I don't really see leaving cross feedbacks for oneself over multiple accounts as being very frequent, especially at those prices. EBay charges a 10% fee per sale (including shipping). I suppose, of course, some sellers might feel feedback is worth $3.90 each.
Search parameters can be tailored to a great extent. If you're looking for a specific kit, you probably won't receive a great number of hits. Simply minus out everything you don't want ( -decals -instructions -parts, etc.) and refine it over time in a saved search. If you look for all kits in general, or 'Ford,' 'Chevy,' 'Mustang,' etc., you'll still get hundreds or thousands of hits whether you add general parameters or not. That's only to be expected. For that reason, I always search for specific ranges only.