I cannot ask the seller to update his receiving address after each sell, that would be way too cumbersome.
Actually that's the way it is supposed to be.
One should always create a new receiving address. For each transaction.
This is done to increase the privacy and reduce the possible information leak (who sent how much to person X).
Generating addresses is not a problem at all. That's basically just increasing a counter and doing some small calculation.
Wallets do that automatically already. And so do most merchants.