Just my opinion, but here it is:
For some reason, these flea market/rummage sale vendors think that by painting over the rust, they will get a better price for the tool. I've seen it time and time again, at least once at each venue I have been to. Evidently, these guys have plenty of time to kill in their "downtime" when they aren't selling, and it just makes sense to them to paint. I also think these sellers are catering sometimes to the "farmhouse chic" buyers that are looking for that special 'hang on the wall for decor/authenticity' purpose.
Then you have some collectors that will paint every tool they display, whether the tool needs it or not. Either it looks better to them, or they think it will impress someone. How many old farm wrenches actually came painted "John Deere Green"??? In that case, since it is their collection, I figure it's okay. They aren't trying to sell them, at least until they die and the family decides it's time to try and make a fortune from "uncle Festus'" junk from the shed.
Another seller I know cleans all the rust off of tools with a wire wheel brush, then slaps a coat of clear on them to "keep the rust at bay" (direct quote!). I buy from him regularly though, as his method doesn't cover up cracks, weld/grind jobs, etc.
Most of the time, though, I find that if it is painted, there is an obviously higher asking price compared to the same rusty wrench five tables down the aisle!!
Enough rambling, I think the others have covered all the various methods most people use to remove the paint. You will just have to experiment and find one way to do it that works best for you.