@lin1876 It works on a calendar system. GT5 counts a number of ‘days’, in which you do things in the game. Doing things like races or license tests advances the game by a day. Each day, a certain number of cars are replaced in the used car dealership in a set order. Certain cars appear very rarely – many hundreds of days apart.
You can use this tool to work out how many days you need to advance for a certain car to show up, but chances are it’s going to take so long to do it that it’s not worth it. Truly baffling design choice by PD.
If I’m not mistaken, online seasonal events don’t advance the game time. So if the Bentley is still in your used car dealership, just grind the Formula GT event a few times and you should quickly be able to earn enough to buy it.