They can be very hit or miss, but are also where you will find the best deals in knives IMO. Some vendors way overcharge for junk, some don't have a clue of what they have, others just want a decent price. When I look, I go by:
Price- is it in budget. Does the price reflect name only? Material type? Special additions?
Completeness- are the scales whole? Blade without chips/broken tips.....
Repair ability- if walk and talk are off, is it because of dirt and use; or is it overuse and neglect.
Can nicks in the blade be taken out easily. If broken, can the blade be repurposed, will small gaps fill in with an oil bath? What is permanently damaged? Has the spring been dented and is now over bladed?
Do I want it?!? Just because it's a complete cheap knife doesn't mean I'll buy it. A stockman will dissappear into my ownership than a trapper will! I'm not a trapper person, so I'll leave it got the next guy.
If you are willing to be patient, clean and tinker with your knife, and don't mind that it's not in pristine condition in many cases, then these locations are great for knives!! Let us see pics of what you find!!
Last bit of advice- believe less than 1/2 of what you are told about the value of a knife from the guy wants to sell it to you!!
All of these were found this way- none over $25 I believe. The stockman was a fixer as both the pen and clip blade were badly broken, but for the price I was willing to put in work to make a spear and manicuring blade!