A FEW THINGS I WISH I'D KNOWN SOONER...
1:  Vials of dye are valuable, so don't sell them to the ordinary merchant - black dye is worth at least 6000 gold, silver dye at least 500 gold, the others anything from 40 to 400, roughly.  After you pass through the Academy to "post-searing" there are special dye traders you can buy from or sell to, or you can trade with other players... when trading with a player, don't click on "accept" until the item or cash they are offering is displayed in the trading window. 

2:  Charr hides are valuable, so don't sell them to an ordinary merchant or salvage them.  The collector Calissa Sedgwick in (post-searing) Ascalon, close to the merchants, will give you one fur square for every four charr hides, and fur squares can be used by an armour supplier to make you better armour, or can be sold to a rare material trader for around 200 gold.  These rare material traders also buy special salvage items such as leather squares, silk, linen, etc.

3:  If you pick up a piece of loot that is "unidentified", you need to have bought an "identification kit" from a merchant.  Double-click on the kit, then click on the item.  If it is "precious", sell it to the merchant, and do that too if it isn't useful to you - check that it isn't better than the weapon you are already using, of course!  If it is "highly salvageable", then you need a salvage kit, again bought from the merchant.  Double-click on that kit, click on the item, and you'll have, instead of the item, some wooden planks, bolts of cloth, bones, or other useful raw materials.  These can be sold to a merchant, or kept in your inventory (an armour maker may need them to make you better armour).  If you want to sell them, post-searing there are material traders who will give you a better price, if you can sell them ten at a time.

4: When you start a new character, you decide right away what his or her primary profession is - warrior, ranger, necromancer, elementalist, mesmer, or monk.  During your time in the first "pre-searing" map you get the opportunity to "try out" all the secondary professions before you need to choose one permanently.  As this involves quests and rewards you don't otherwise get, why not try them all?  Do every quest you can (signified by a pale green exclamation mark over an NPC's head) before you take up the offer from Sir Tydus and enter the Academy - once you enter, there is no going back, and you'll soon arrive in the main, larger, "post-searing" world.  Some quests give you money, some give you special skills that you would otherwise have to buy from a skills trainer - and they all give you experience.  You shouldn't enter the Academy until you are at least level six, or you'll find your early post-searing quests rather hard, and won't have enough cash and materials to get good new armour!

5: In pre-searing, you can only expand your available inventory size by obtaining a five-square
pouch
to go in the second slot - the collector Brownlow just outside the gate, by the resurrection shrine, will give you one in exchange for five skale fins, and other collectors offer them as well.  In post-searing, when you reach Yak's Bend the merchant there will sell you the two five-square
bags
you need for the third and fourth slots, for 100 gold each - or a guild member with access to that location will be glad to fetch them for you.  Any merchant will sell you a "rune of expansion" for 500 gold - double-click on it, and click on the bag, and it will expand to ten squares.

6: The two armour sellers in (post-searing) Ascalon offer different grades of armour - Banoit's is cheap and not as good as Corwen's, so try to get armour from Corwen if you can afford it.  Some collectors offer armour in exchange for items like singed gargoyle skulls, ornate grawl necklaces, and fetid carapaces, but it isn't as good as Corwen's armour, which is the same type as offered in Yak's Bend, so will last you a long time.  Other later outposts offer better armour - the Crafter just outside Sanctum Cay is well worth visiting before you go to the crystal desert.  Just leave Sanctum Cay by its portal, and keep close to the wall on the left and you shouldn't need to fight anything, you will soon reach him.  His armour, though the choice is limited, is very nearly as good as Droknar's Forge armour, and better than any you'll find in the desert itself.

7: By the time you reach Piken Square, you'll be able to buy an expert salvage kit as well as the regular one - this is the one to use if identifying an item shows that it contains a rune or other useful upgrade (such as a bowstring, staff wrapping, or sword hilt).  It also increases the chance that you will salvage a rare raw material such as a steel ingot, bolt of silk, sheet of parchment, etc.  Once you have a fair amount of spare cash, maybe by Lion's Arch, it's probably best to stop using the cheaper kind of salvage kit altogether. The rare material traders will buy steel ingots, leather squares, etc, singly, if you are sure you won't be needing them for an armour upgrade.

8: Get your weapon customised by a weapon trader, as for 10 gold it adds 20% to the damage you can do  - unless it's very valuable and you don't expect to use it for long.  Only you, now, can use that weapon - be careful if you buy a weapon from another player that it isn't customised for him alone!  You can still sell a customised weapon to a merchant, however.

9: You can only use armour you buy in person from an armour maker/seller, or exchange items for with a collector.  It is automatically customised for your use only, so you shouldn't try to sell it on afterwards.  If you have added a rune (only ones for your primary character type, or improved vigor), you can probably salvage it for re-use, using your expert salvage kit.  Armour you pick up as loot can also be salvaged, or sold as loot - if it is "unidentified" you should get a rune from it, which you can sell to a rune trader or another player, if you can't use it yourself.  Runes don't "stack", so only use one of the same type; if you've used a minor rune on your equipment, and then get a major rune, you can "overwrite" the minor one with the new one.  Many runes, including major or even superior ones, are only worth 25 gold, as the penalty in health points means many people don't bother going beyond the minor runes, which are fairly plentiful.

10: The "alt" key is useful for helping you to find merchants and other traders in towns, as it brings their names up - out in the countryside it will help you locate dropped items, chests, and NPCs such as collectors.  The "ctrl" key brings up player names in towns, and outside it brings up the names/types of each monster, very useful if you see red dots on your compass just round the corner, but don't know how many there are, or their type!  If a player says they are making up a party for a quest or mission you want to join, but you can't see them, either type their name in the box under your name, or copy & paste their name across from the dialogue area, and click the little "+" button alongside. Keeping the ctrl key depressed and clicking your green exp-bar, equipped weapon icon, etc, makes your character announce "I am level 16 and 37% of the way to gaining my next level", "I am using a furious hammer of charr-slaying", "I have 43% death penalty!", or whatever - keeping it depressed while you use a skill on a monster makes you say "I'm using vampiric gaze on Rudy Footknocker" (or whatever) - and anyone else who hits the "T" key and space bar will automatically target the same monster.  ctrl + shift + "H" hides the HUD if you want a clear screenshot ("print screen" key), the same combination brings the HUD back.

11: The gold "rare scrolls" you can get as loot can give your party extra experience for a certain time - or any trader will give you 100 gold for them.  But look out for special Rare Scroll Traders (such as the one in Amnoon Oasis), they buy and sell these items, and, though the price will vary, you may well find they will offer 600 or 700 gold for a rare scroll.  They aren't interested in non-gold scrolls, however.
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