I don't sell online, via a website, on ebay, etc... I will occasionally sell a book from inventory to someone that inquires, but that is not my normal modus operandi...
I generally only sell "wall" books at conventions (I keep some kewl under $1000 books in the store, like Walking dead 1, or tec 359 or ASM 129, etc, but that is only for the occasional out of towner...locals tend to like to just buy new books or cheap back issues for entertainment)...
I buy books year round. I don't have a secret source to procure them under FMV...I generally get the majority of my inventory from auction houses and other dealers, and then just cycle books into my wall/con inventory as I believe they have appreciated...
My typical mark up is single digits to maybe 20% on raw bronze (just general, always exceptions)...
My price is my price, regardless of what someone else is selling for (ebay, another dealer, etc...)...I can't do anything about what I pay for a book, except wait till the market will bear the price I'm asking... and I'm ok wanting folks to purchase a like book elsewhere for cheaper, if they can...that's good for them and doesn't hurt me at all...
my model is not typical, I don't believe... I could play games and mark the books up an extra 10% (or whatever %) so that I can "discount" them that % , but that seems like a lot of work to me, and many times at a convention, I just don't have the time to constantly "negotiate", because often it is just me (my wife and son are generally there, but their vintage book knowledge is limited). I like taking my cost, putting a fair mark up on the book, and sell it for what it is marked. The resistance I encounter is that traditionally, most sellers build in some margin to discount at a con...I don't... many times the book I have for $200, is $250 elsewhere, so the "discount" is already built in....sometimes I am higher than someone else in the room, but my 4.0 might "look" better (or have better pq, or some aesthetic bonus)... sometimes I just pay too much and am just higher for no other reason... many times I am the only one in the room with certain books, and I get no resistance from consumer... just a myriad of different examples...
so, match ebay? my answer is a simple "no"...