My question as a seller is that I currently have my postage separate.

 I do not included postage in the cost of the item.

If a buyer who has an ebay plus subscription gets free postage who pays for the postage cost to the seller.

Does ebay pay the postage cost to the seller and if so how do they pay it.

i.e. If ebay covers the difference on postage cost for the seller does the seller get the money at time of sale or is it taken off their fees.