Some cars always sell well second hand.... but only if you pick up the right one to begin with.
The usual Golf, Polo, Civic etc etc (pretty much any of the most popular makes) always trade well and you can find some in very good condition.
Importing from the north can be very fruitful, again, if you find the right car. Some of the older BMWs (96 or so - older car keeps the VRT lower) can be picked up for a bargain across the border.
Some models seem to sell very well when they hit the market due to a slight lack of supply. I've had a friend looking to pick up a good Mazda MX3 and has been unable to find any anywhere near in decent condition (he has decided to begin looking in the uk to find a hopeful bargain).
To be honest, if the car is at the right price it'll have no problem selling whatever it is.
Try and pick up a good deal, and you'll not lose too much on it later.