I have done both and will never boot again. At the boot, the haggling is done down the way, so you suggest a price and the cheeky sods argue you down.
On ebay, the price goes up.
Obviously you have the hassle of going to the post office etc, but most boot sales start at silly o'clock in the morning so that's a hassle in itself.
I have never successfully sold books at a boot sale.
The only bonus of a boot sale to me is having the actual cash in your hand at the end of the sale, it feels very tangible. But looking back, I could kick myself for selling stuff for a few quid at the boot sale before I discovered ebay.
It's true, there are some brands where apparent logic goes out of the window and ebayers seem willing to pay almost full price for used items. Boden, Joules etc are a case in point.
I think many people shop on those websites thinking 'I'll get most of my money back anyway, on ebay'!!