People often refer to scamming as:
Trading someone hacked Pokemon. While this one's position as a "scam" is debatable, it is still considered bad since some people like to keep their game free of hacks and play legit, and because of a troll or scammer, they may receive a hacked Pokemon.
Not trading back after promising to do so. Sometimes people trade to evolve their Pokemon, but unfortunately some people won't trade back and will keep the Pokemon. This could also happen if a person lets the other person "borrow" the Pokemon for whatever reason, and the recipient refuses to give it back once they are done. Another possibility is that scammers lure others into trading by offering to do things like level then with Rare Candies, teach them tutor moves, give them back holding a certain item etc. but they keep the Pokemon instead.
Stating that a Pokemon has something but it doesn't. For example:
Guy 1: I'll trade you a perfect IV Charizard that knows Dragon Dance if you give me your Chesnaught.
Guy 2: Really? Thanks!
Guy 1: No problem.
Guy 2: It doesn't have perfect IVs. And its moveset is stupid... it doesn't even have Fire Punch! Give me my Chesnaught back!
Guy 1: No, sorry I gtg
Guy 2: :C
Something like this or something similar usually.