If you use SD - ebay will be paying out if they go missing or are damaged in transit - it is a much easier system with ebay - quick and no proof of cost or loss -
Now although that is a really good thing for both sellers and buyers - because the seller keeps the sales proceeds and the buyer gets their money back quite quickly, I suspect ebay have had to introduce this as a carrot to sellers and because:
As a business user of RM they will not get any insurance for lost or damaged items in transit nor any refund of postage costs - this is standard practice for RM business services - what they get is averaging and cheaper prices - ebay will have used historic information to how much they add to each postal price to cover this insurance from themselves
The postal agreement is between ebay and RM - you are not part of that agreement
A big bonus is that private sellers and buyers no longer get into stupid often heated discussions on who should claim from RM, whose fault it is, - ebay take all the responsibility and simply refund the buyer and let the seller keep the sale proceeds - everyone should be relatively happy
