between you and seller,seller is the owner.

Between lender and you, you are owner. So for  normal transaction,  both of you need buy.

It is sellers responsible to buy one for you. And you should get a copy of the title insurance doc forward by escrow company 

The title is insured, but not that you pay, seller should pay, that is the normal practice. 

Different state may differ? At least in California that is the way, actually in California,  you don't even need ask, escrow company will ask seller buy that. You Just need tell escrow you don't have loan, so they don't buy for lender and put fee on you closing sheet.

Again, it is not that buy or not buy, it is who pay for the policy? In your case, seller pays for the policy, fee should be on seller's closing sheet.

In normal transaction(with loan), insurance company sell the same policy twice, that is How they make money!