Can buyer of Steam keys claim your account? and he have proof os purchase of his key?

Well, I'm not really sure, but they probably could, since Steam Support is dumb as crap (it's miracle if they read half of what you wrote). If someone contacts support and posts the key, (tho, like you said, they must know his username), all online activity on his account will be closed, no chat, no key activation etc until it gets solved, in that case, OP would have to provide different proof.

They could simply claim the account was hijacked, I suppose, but they might be able to see that use of the account appears normal. i.e. since when do people hijack accounts and not change the email/password? Why would the hijackee be emailing from a completely unrelated email address? etc.

+1

Yep. In 8 years of service I've never once logged in to my account outside of Southern California, and maybe two or three times outside of this house. I've only ever used one credit card as well. The more you can keep your use consistent and predictable, the easier it is to reclaim a stolen account even without evidence like key scans. When my account got hijacked last month the night and day difference in account activity was so jarring and blatant and I got my account back in a matter of days with minimal back and forth with support.

So the question then might be what happens if I purchase something with my current card (which we'll assume is about to expire,) and I shred it when I get a new one so I no longer have that information. Can I no longer prove that I in fact owned the credit card I paid with?

This sounds plausible, hope i don't have to deal with this situation, though

In that case support most likely will ask current keys holder (one who activated them) where he got them from, and answer like "I got it from user XXX/emial/link to steamgifts profile" will probably handle that case in your favour - proof of buying is only proof of buying not proof of owning it.

You can always provide proofs of the rest of hundreds purchases you made on your account, can you not?

i think you need to write more information to take back the account, only a proof of purchase is not enough if you don't know your associated e-mail adress and login id they can't do anything...

this

And if you don't buy anything on Steam - only retail or other cheaper stores? Your account is gone?

If you win a giveaway of a key I've purchased I have proof of purchase and your username, nick and maybe e-mail. No hijack (but not acces to e-mail) so you are on hands of the staff decision.

6: Sell accounts while retaining enough information to tell them apart
7: Have the capability of getting any person who annoys you banned forever

How do you activate a key for a second time if I may ask?

That's funny. I hope you got your money back, but the seller deserved it

"Hey, here's this beer I already drank most of, but you can have the last dregs for real cheap!"

Disabled for life, or just until it gets solved? Since this can be really bad precedence.

so you and that guy both lost something on the process, right?

maybe it was one of those key generators?