Not at all. It's a personal decision made between two traders and nothing more. GT5 recognizes only limits to trade:
5x paints per day
1 car/ticket per day
You're not obligated to trade your car for paints (or the other way around) but if you want to, like I said, it's your decision 👍
The "value or worth" of the car can be expressed by how "rare" is it to find in UCD (Used Car Dealership) and some might offer anything from just 1x paint chip up to a level/birthday ticket but then again that's just my personal opinion.
It's really up to you to strike a fair deal.