A trade can be months old and valid, your argument fails.

Backups are pretty fast as they are, so I'm not worried about that.

Trades that are not bumped expire after 1 week already.

We rely on donators to turn off autobump on their own trades.

Instead of creating a new post, I thought this was rather close enough to continue on. I would suggest for those donators with autobumps be required to do something to maintain autobump status. For example, I was searching for a particular item when I came across this user:
http://www.tf2outpost.com/user/190984
In one of their trades they mention "add me for quick trade!"

However, its been over a week and they have not accepted my friend request. I have shadowed the person's profile long enough to know that it isnt an abandoned account; they have periodically logged in and out. The person's backpack has also stayed the same throughout this time even though through comments, other people have also attempted to buy other items from the same person.

I understand its basically a perk of donating but there still should be some method (perhaps a button for donaters that requires being clicked on to maintain autobump status for, say, a month; that doesn't mean they have to go back every month to click it, they can just "refresh" it at any time) for requirements to keep it. It appears this persons was last active (on Outpost) in February as they have not replied to any comment (maybe around 50) on either of their "active trade".

I don't know why they have not accepted my friend request but I don't think we can rely on them to turn off autobump.

I'm a donator and I'd love to see this. Just require some interaction with the site (post a comment, edit notes, bump a trade, hide a post) once a week or whatever, and disable autobumps otherwise.

Now that users can re-open trades, I think this is a viable idea.