How long did it take you to decide to sell your car

So.... I'm about to sell my 68GTS. What's the best way to determine what it's worth? I've done the search and it seems prices are all over the place.
That's my problem I have used this site to sell several cars and they always went far away so you don't have to see or deal with them Cars On line.com: Classic Cars For Sale (cars-on-line.com)