it can be very hit and miss. you can put it on a buy it now, but it may not sell. on an auction, it will sell as long as your starting price isn't too high. you might get less than you expected, or you might get more.
you can put a higher start price and / or reserve, but these can tend to put bidders off.
i'd advise you to put a buy it now with a "best offer" option.
as advised only accept cash on collection, NEVER paypal.
Gumtree is also worth a shot (and free) , but you get loads of would be scammers / timewasters (ie. people offering you ridiculously low amounts before they've even looked at the car), but as long as your car is realistically priced you usually get a serious buyer.
The Autotrader site is usually worth a bash too (think it's £30 for an advert), but you get a lot less time wasters in terms of people offering you 2/3 of the asking prices and will pay "cash". (tho ignore emails from scammers who will send you messages wanting to pay by paypal and send their mate /a courier to pick it up)