No.
Why would it be rude to get a feeler for what vehicles are buying/selling for?
Now it would be rude if he came back and said "shouldve gotten more" or "shouldve paid less" for something, but a simple inquiry isnt rude at all IMO.
How much money do you make a year?
People can get a feeler doing the research themselves. Log into eBay for a few minutes and you can figure out the prices on cars. If you are a true enthusiast, you already know what the vehicle is worth. If you have to ask you either can't afford the car or you are not serious about buying.