I always find this a very curious one, and frankly it's stupid of a seller not to apply feedback, my preference has always been straight away.

You enter into a contract with the buyer, all the buyer needs to do is pay, that's their side of the contract done.  So why aren't you leaving feedback at that point?

By putting feedback on the buyer you're now advertising your sales and/or business to anyone who checks feedback..... since around only 25-30% of buyers bother to leave feedback anyway you're mugging yourself off as a seller not to get 100% coverage on your buyers by leaving 100% of feedback for other people to see.

You know how I can tell if someone's bought something off me 10 months ago....I scroll down their feedback and look for my name.  You know how I can tell if someone who's appeared on a competitors feedback has bought off me too....I scroll down their feedback and look for my name.

Sellers aren't always sensible business people, in fact some are quite ignorant to maximising exposure and small tricks to drive extra sales, I like that, it's less robust competition for me to worry about.