That's an interesting point. Is eBay now considered a "wholesale" place to sell one's collectibles? Somehow I thought it ranged between retail and wholesale, leaning toward retail, if the piece was rare...(and maybe that's what you are saying...).
Nope, but ebay will always sell for a lower when there are copies on abe, unless the seller is a moron. If a book is on ebay, and hits $250, when there are 5 copies for $200 on abe, then there is someone that wanted it so bad that they did not care to look at the first place to look when deciding on these things. If there are no copies on abe, then it can be tough.
So, given that, if the book is not truly rare (as opposed to the tag "rare" that is always put on anything that the seller wants to sell), the the ebay price should always be below the abe (bookseller) price. Add to that, that most booksellers will give discounts to good customers and you can see why the "spot" price on abe should almost always be the highest that you should expect to pay for something.
In my opinion, anyway...