X.5 Y Keys, what does it mean?

It's a least 2 keys for crates in CS.GO and the half price of a key.

For example, in TF2, it's the same principle: 2.5 keys = 2 keys + something worth half-price of a key (whether in metal or in items).

It means 2 treasure keys and items worth 0.5 Key.!

It is the cs:go key: http://steamcommunity.com/market/search?q=case+key
Half key is usually the capsule key: http://steamcommunity.com/market/search?q=capsule++key

Thanks guys.
So if I buy them two case key and one capsule key, and they agree to trade, how do we actually perform the trade?

seems like you already got your answer. welcome and please don't get scammed by an impersonator.

If the men offer you candy, don't get in the van without trying the merchandise first.

Be really carefull who you trade with , if you are new to the whole thing.

There are quite a few scamers who just love people like you :x

I have noticed some men in the trades category

They might be women ya know.

The usual currency for 0.5 keys are Tour of Duty Ticket(TF2) & Community capsule keys (cs:go). But they are acceptable only if stated so.
2 x of those items don't count as a 1 key. :))

Ofc anything marketable for ~ price of 1/2 key can be accepted e.g skins , sack of gems etc.

Always agree with the other person about what exact items will be traded. Not everyone has the same opinion on what .5 means.