I am not likely to be able to make a recommendation, as I am likely not in your general area. I can help you a bit though. There might be forum members here in your area. If you post what area you are from, what kind of uses you are looking at and a rough idea what budget you are working with if anyone that can help you sees this they will be much more likely to respond. Really, looking in the used market you want someone as close as possible to respond.
I have bought some used gear off of the internet, but I am still personally leery of that.
Where I am at in Oklahoma City, Ok, USA. Things like that do show up from time to time in pawn shops.
Any amps you find in a pawn shop you can have them hook them up and demonstrate for you. You can also write down the info on them and look them up on the internet.
While working with a budget it is also not a bad idea to know what budget brands are pretty workable.
I do not recommend everything they make, but Behringer has stepped up their game as far as PA gear in recent years. I own an NU4-6000 which has proven to be a rugged, reliable and versatile workhorse. It sold new for about $350.
I have a friend that used to work for Crown/Harmon/Selenium (I am not sure how they are names are supposed to be written). He has a pair of NU-3000 DSP amps. That is basically the same thing I have except his is separated into two chassis plus they have all the dsp functions. He tells me he opened his up to take a good look and the INuke series amps are a good design and use good parts.
I don't find my NU4-6000 on Behringers page currently, so they may have stopped selling it.
The do show the NU-3000 and NU-12000 (non dsp) as well as the NU-1000dsp, NU-3000dsp, NU-6000dsp and NU-12000dsp.