There are tons of good non folding bikes. There are tons of good used non folding bikes cheap as you have found out---a bit of patience and you can find a good bike cheap.
There are very few good quality folding bikes sold. $800+ when new. The folding market is really focused on Cheap bikes. That is why there aren't many good folding bikes on the market used. There are a few brands that are very sought after---Brompton, Bike Friday, but they never seem to be for sale for less than $500 and that is if they are beat up.---The Craigs ad above for the bike friday at $350 is really rare.
I have a swift and a fairly nice Dahon Mu D10. I added my favorite bar ends to both of these bikes. It is very easy to adjust the handlebar height of both of these bikes while on a ride---It takes an allen wrench on the Swift but 15 seconds and I can have the bars at a different height. Most of the more expensive Dahon's have adjustable height bars with a QR adjustment.
Good luck in your search---I believe you are in a tougher shopping market than you are used to in the non folding bike market which is overflowing with bikes people want to get rid of.
Last edited by Rick Imby; 09-03-17 at 02:54 PM.