A question of etiquette

Well has that person been on? Have they posted in that thread? Either they forgot or are ignoring your offer imo. But you got nothing to lose to add them and ask.

Extremely bad form.
Generally, people don't like to be pestered by individuals they don't know.

Moreover, if they've got public comments, that implies it's okay to message them on their steam profile.
If they don't, it implies they like to be left alone, in that regard.
There shouldn't ever be call for blind friend requests..

Mostly people are annoyed by that behaviour and may block you instead of accepting the friend request. If the person has their profile comments open, post there, then you can add him/her; that is a more frequent way of contacting a trader.

As long as you're polite.

Thankee for the replies! Sounds like I'll be adding the one with a bijillion comments on their profile and just...waiting on the other--though that one at least hasn't been on Steam in several days so makes sense.