I was looking at a new PRS 505 on eBay and the seller advertised it as coming with a "soft case", Would I be correct in assuming this is some sort of slip case supplied with the device, and not the Book style cover.

Why not ask the seller?

I could ask the seller, but I wanted to hear from someone who has bought one, and if the case ships as standard, what sort of quality it is.

The one I purchased from Ebay came with a tan case, I guess I'd call it more like a cover for a day-timer than "soft". In any event it's very nice.