I'm in a different place from the OP, because I never buy a book to "flip". I know it doesn't 'work' on the majority of sellers, but I just communicate that it's for my personal collection, and be as nice and polite as possible (impossible, right? Me, nice?).
I buy a decent amount on eBay, and I know that sellers have to compensate for fees, but sometimes the markups are excessive IMO. I float my honest Best Offer and include a note that it's "honestly the most I can offer, not trying to haggle".
It doesn't seem to work in-person. I once tried to buy a DC Encyclopedia from a guy (priced MSRP $40/50), but the binding was completely destroyed, and I have to re-glue it. Wouldn't budge below MSRP. No one will ever buy it for a brand new price when it's destroyed...