If anything like Gumroad, two options.
- Each user has a randomly generated purchase code, the seller relays this, the seller checks if the code is in their order history, then go from there.
- As each is purchased, the seller receives a name/email/something to tie the two together. Gumroad’s case, each purchase sends an email stating XYZ@ABC.whatever email has bought X product. Email address is otherwise used like option 1 above.