Advice?

then sushisu's suggestion could work, if you got the key from a bundle site that can revoke keys.

if not, send a request received feedback ticket with proof you sent the key and where you bought it (screenshots). not sure if this works but it's worth a try.

it was a GA that ended about 10 days prior to the GA in question that made me curious

Have you checked the user now? It was a GA that ended about 10 days prior to your GA that made question this.

Best you can do is try to contact the winner to find out what happened, can't assume it was them. As stated above contact HB letting them know the issue you had with the key.

No - I make it a rule to trust everyone is doing the right thing.

Don't make that mistake. You'd be surprised how many shady individuals there are on SG.
If you're cool giving some people games they're just going to sell/trade, then by all means, disregard this message.

But also, keep in mind ... every person you report for rule-breaking on this site is one less headache for the next guy, and you're helping the entire community by reporting them.

I make it a rule to trust everyone is doing the right thing.

There are thousands of rule-breakers here, you should always check your winners. For example, your winner has:

1. A recent VAC ban.
2. One not activated win (from this same month)

So probably he's trying to break the rules again. And judging from his profile info, he's not exactly a child.

Having to much trust can backfire because besides good people on SG there are also some very bad and useless ones that will not play by the rules and/or don't care about the rules. I know this out of experience making several hundreds of public giveaways that were from level 0 up to and including level 3. I had winners with multiple non activated wins, winners that marked received and re-gifted, winners with multiple double wins, exotic ratio's, VAC and Game bans, even one with a trade ban.

Now I've transitioned my public giveaways to level 2 SGTools giveaways and it's much nicer to check my winners and to give my gifts to SG users who do follow the major official SG rules and who aren't cheaters and the like.