How do I hold a draw or lottery that requires proof of purchase?

If I want to hold a draw or lottery that requires proof of purchase, is there any way for readers to prove they downloaded and bought my e-book?

Or bought the print version?

Is there some way to announce and arrange that the winner will be whoever gets the most people to buy my book because she recommended it on her site or blog or just personally recommended it to people?

I suspect no one has come up with a way so far, or more people would be doing it. Can we brainstorm to think of ways to do all the above? It might help many writers who would like to try to increase sales this way.

My novel is a romance where the heroine falls in love with a prince.

Do you think having the prize be a princess doll would sound silly? So far I've mostly seen prizes be Kindles or free e-books or print books.

By princess doll, I mean a doll about the proportions of Barbie [but not Barbie], dressed in a ballgown.

Years ago I saw Cinderella, Sleeping Beauty, and other fairy tale heroine dolls advertised in a British magazine, and they were advertised to adult women. So I assume women still like to own such dolls.

I've tried to think what else romance readers might like as prizes. I suppose jewelry, but that is so subjective. I might love a piece that others think over the top.

Ditto clothes. I thought perhaps a sequined top, but again it might not be to everyone's taste.

Thanks for any suggestions.

How do I hold a draw or lottery that requires proof of purchase?