Just simply the UE4 username should suffice. Maybe not even that- just being able to do a check like
“Has [username] purchased my pack?”
that is just a box for the username, a dropdown for packs/assets you’ve made, and a submit button with a simple yes/no result would suffice.
Sellers can ask for a username if they’re contacted by email, and if they were contacted by multiple emails claiming to be the same username, they can simply PM that user on the forums for clarification.
“Oh god, he has my username! I’m screwed!!”
Well that’s a real shame because by posting anything to a public forum, it becomes publically accessible anyway. By asking for support on the forum, I have your username. Why should I not be able to check “Does ‘user’ have my asset”?