What's preventing the other player from just refusing to trade it back?
If they absolutely do not care about anything except making you suffer, and you're too far away to threaten them with violence, how can you be sure?
You're opening yourself to betrayal. I could never do that.
>hurr durr trade with a friend
People theoretically can pretend to be your friends for a long time just to drink your tears. That is a choice they are able to make. It's not likely, it's not efficient for them, but you can't read their minds.
>hurr durr ask for something rare in return
If they are determined to make you suffer, they don't mind a losing deal. They don't want the pokémon you're trading. All that matters to them is that you don't get what you want.