I purchased a game through a retailer, finished the game, then traded the game in, I then tried downloading the game through the PS store around a year later when it was super cheap and it wouldn't let me download it, can anyone tell me why? Cheers
2 cases here:
If you have bought the game again on the PS store but it won't let you download it, I suggest contacting PlayStation Support to fix the issue. It's probably rare for players to buy a physical copy of a game, trade it in, and then re-buy on the PS store.
If you haven't bought the game again and are just trying to download it (I'm assuming it is still marked as owned as you used to own it), then you'd need to purchase the game from the PS store before downloading it again.