I literally had a guy who showed off his shiny legends, and he was missing some that I had, so I showed them to him and he wanted them, putting up the same messages your guy did, Kayfarron, and I cancelled my end of the trade and the last things he put up were a big eff you in all caps and then a bunch of messages that said I suck and all. People are just stupid and mean about it, so I'm always weary when I accept trades from Passerbys.
But never accept them to show up in your friends list. I had a guy ask me for Deoxys and kept putting up the SUM2013 Giratina. I said no and that it wasn't for trade, and then he kept asking to trade and asked for the same pkmn and offered the same pkmn. He apologized and then wanted to register me so I did, and all he did was spam the same trade over and over again -.- So I tried to stay offline for as long as possible, b/c I didn't know how to remove him from that list, and he stopped asking after a good 2 weeks