Yes, you can use best offer. It's your store.
Yes, you can use immediate payment required.
Yes, I 100% agree with Michelle, it's a hassle to the buyer and it doesn't matter whether you're using ebay checkout or Auctiva or you handle it yourself...because......
When your customer HAS to/is REQUIRED to stop and pay for each item as they buy it...that means the item plus shipping costs, they can't very well stop buy 2 or plus items and then combine it to receive the shipping combo price. Or wait to combine shipping because payment is required immediately before they can buy another item they have to COMPLETELY pay for the first one.
I know, I had to do immediate payment required once and it's a pain in the patoot. I don't like it because it keeps most buyers from buying more than one item.
Take care, Donna