Test driving a private party car you have no intention of buying?

Contemplating the ethics of this... I'm looking to pick up a proper sports car and there are various models I'm considering. Reading reviews and opinions online can only tell you so much, so I need to actually drive some cars to narrow down which one I actually want. I'm looking at used cars that are too old for CPO models so I can't just pop into a dealership.

The problem is I live in a small city, and while I can find examples of most of the cars I'm looking at, they're not the actual specs I want (maybe too high miles, maybe auto instead of manual, standard instead of upgraded suspension, etc). But I'm also not keen to travel hundreds of miles to try and find an example of the actual car I want to buy to compare the various options.

So is it ethical to go test drive someone's private party car I know I'm not going to buy? Should I be up front with them about it, maybe offer $20 for their time or something? Or just pretend to be interested and then say I'll think about it? I hate to be a tire kicker but I don't see what other options I really have here...