Personally I wouldn't buy a used canister, as they can be touchy. If someone didn't take care of it well, you would need to replace seals, impellor, or other parts. Then you are close to the price of a new one, so I would just rather get a new one. I like the SunSun's and seeing as they start at around $40 including shipping to your door, you can't get too much cheaper than that.
If you want to look for them, craigslist sometimes has them, but more often they have full setups from someone wanting to get out of the hobby. Again, if they are getting out of the hobby, odds are they weren't taking care of their equipment.
Look on club sites, they're pretty reliable. Guys won't sell junk, word gets out on that & the locals will never let them live it down. Often getting out of the hobby is due to a medical issue, 18 months back I had to help sell off a 30 tank fishroom with another 30 spare tanks, and all the associated gear. Guy decided to have a heart attack, check out early.
I personally don't have a problem buying used equipment and I've had good luck with used filters. You should be able to tell pretty easily if it's been cared for properly or not. I just picked up a used Filstar XP4 for $75, over $100 off the price of a new one, and when I got there the guy had it set up running in a 5-gallon bucket so I could see that it worked. That would be the ideal situation- to be able to see the filter run before committing to it.