TF2 trading crash-course:
Ref = refined metal = 3 reclaimed metal.
Rec = reclaimed metal = 3 scrap metal.
Scrap = 2 craftable weapons. When a weapon is "craftable" that means it can be "crafted" with another weapon into scrap metal. Craftable weapons must be usable by the same class to be crafted into scrap.

Keys = ~7.5 ref right now. Keys are used to open Mann Co. Supply Crates, can be bought on the Steam Market or in the Mann Co. Store.

Buds = earbuds = ~19 keys right now. Given to Mac users that played TF2 on their Macs when the TF2 Mac update came out. Used as a general currency, like keys.

Bills = Bill's Hat = an expensive cosmetic sometimes used as a general currency. Given to players that purchased Left 4 Dead around its release.

"S>item" = selling an item. Used as shorthand, especially in names and in-game chat.
"B>item" = buying an item. See above.

"B/O" or "BO" = "best offer", means the seller/buyer is looking for this price but is willing to negotiate.
"C/O" or "CO" = "current offer", means the seller/buyer has been offered this price and you'll probably have to beat it to catch his attention.

Item levels matter to some people, it doesn't always modify the price of an item.

Craft numbers can and usually do change the price of an item; the lower the craft number, the more desirable the item. You may see #1 crafts sell for multiples of the original price of the item.

Honestly that's all you need to know.
Some good sites to use for pricechecking:
http://www.backpack.tf/
Steam Companion[www.steamcompanion.com]

Obviously since you're in this group you know about trading websites. There are a lot more than Outpost, though, and for a lot of different purposes. For example, http://www.scrap.tf/ specializes in automated scrapbanking and robot item banking, among other things. FYI: Scrapbanking = trading weapons that normally can't be crafted together (i.e. a Heavy weapon and a Sniper weapon can't be crafted together to make scrap) for scrap metal.