There's nothing malicious about a random friend request. I send friend requests to people I enjoy playing with on pubs so that in the future I can jump onto a server knowing that I'll have fun.

If you're not actively selling items on TF2TP, SourceOP, etc. and get a friend request out of the blue, they've probably used one of the methods available to search people's backpacks and found an item they want in yours. You can accept it if you like, but expect them to treat you like you don't know what you're doing and try to rip you off. Hey, at least see what they want. If they're a jerk, just un-friend them, no harm done.