For the reasons cited by HeRetiK. I searched for "bitcoin xpub key" and some of the first results warn about sharing it.

I agree that asking the seller to share his xpub key shoudn't be an issue if a wallet specifically made for my website has been created by him.

Asking for a batch of pre-created addresses also looks to me as a viable idea.

I may even give the choice of the method to the seller...

Thanks for those ideas!