Usually I ignore random adds, and so do a lot of people. Etiquette is generally to comment somewhere so people have an idea of why someone is adding the user, or people just look suspicious.
Some users will just block random adds too.
edit: you shouldn't accept random adds either. it's how people end up being scammed (but mostly it's because they never check the comments for who posts to find out it's someone else)
you should add in the items to your have and want list so people who search the forums can trade with you easier.
Most of it was just information if you want to trade with people. I only wrote it here because I figured it was probably trade related.
edit: also part of it was just to say not to accept any friend requests. you won't know exactly who you are dealing with that way. it's kind of scary unless you don't care. I wouldn't want to lose items, at least.
double edit: scammers aren't very gentle either. they'll take your items and then just move on to someone else.
Send friend request on steam