It may not be as popular as a Focus or Astra but there are a fair amount of Mazda 3s around. There have been a few special edition models such as the Katano and Sakata and these are worth going for as they have extra kit plus unique alloy wheels and paintwork. The 1.6-litre diesel is the most popular model and will command a premium over the petrol equivalent.
Unless you have a base S model, the Mazda 3 should be a simple enough car to sell, plus if you have a diesel model then it should shift in no time. The Sport models are sought after because of their more aggressive looks, but the later Takara also comes with sporty looking bumpers and a rear spoiler. Be sure to emphasise this if you're selling one.