Sid, I think you're right about buyers buying one specific item - I have found that as well.
When I'm buying, I look at the seller's other items if they combine postage, and am more attracted to sellers with a good range of items on offer.
But I was trying to buy several items from one seller last week and asked them if they combine postage. I got no reply, despite Ebid automatically nudging 6 times! Since the items were needed for this week, it's too late now and I bet that seller is wondering why their stuff isn't selling!