Depending on what you have, gomotorbids.com might buy your collection only if they are mostly 1/24 scale. If you try sites like this or eBay, you're probably better off to sell them individually or in groups.
Prices were way down on 1:64 scale cars at Speed Week in the Daytona Flea Market. Dealers/Sellers were still asking a mint on the 1:18 scale cars though. I did pick up some bargains on the smaller cars.
i've had luck selling stuff local at a better price. there are many free ways to advertise if you check around. we can in the newspaper here place adds for $100 dollars or less free. and a t.v. station has a free classifieds website with almost 20,000 members. and i've sold things to people from texas to new york to oregon off this little website here, and gotten really good prices. also we still have a party line on a radio station for selling items and sold alot on there. some stuff 2 or 3 times what i paid. that lets me throw away other things i can't sell.
i would try advertising locally before selling on line. really advertise hard in as many places as possible.
as was said above, selling it all to one person is impossible without taking a huge loss. i've just done it a piece at a time mainly, and have been able to just throw away alot of other stuff from the profit i've made locally..