I've purchased three tanks off of craigslist, a 75g, 29g, & 5g. All three have been working fine. The larger two came with cabinets/stands & accessories that made them affordable for me over purchasing them new. The two larger tanks were in use when I first viewed them so I could verify that there were no leaks or other problems. I was able to keep the filter media and gravel and begin to stock the 75g right away after purchase. No need to wait for a cycle because I was given the beneficial bacteria.
1. Expect to give the tanks a good cleaning, usually people offloading their tanks just don't have the time for them anymore or need the money.
2. Wait until you find just what you're looking for. The size, style, etc.
3. Make sure you get the history on the tank. If the person says "I've had it 3 years." I would then ask, "Did you buy it new or used?" Make sure that you're comfortable with their answer. I don't want to purchase a used tank that's ten years old. Eventually they will break down and I don't want a surprise, especially on a larger tank that springs a leak after 6 mos or a year.
4. They may want you to take extra supplies and/or fish if they're "cleaning house". Take them. You can always sell/give them back away on craigslist or to friends. I recently took a
Clown Loach when I purchased a tank. After researching the fish's needs, I determined I really don't want it. I'll either pass it on to my lfs or rehome it off of craigslist. The guy gave me a good deal so no problem.
5. If your gut tells you something's wrong, don't go through with it. You have a right to inspect the setup, ask them to put the tank on dry cement/asphalt and fill it with a hose so you can inspect for leaks. Don't expect perfection, but it must be in useable condition. Like anything else purchased used, buyer beware.
6. If you're a non-smoker and are buying a tank with a cabinet, you might ask if the furniture is from a non-smoking house. Smoke smell gets into the wood (the tank can be cleaned so that's not an issue) and is hard to remove, especially if any part of the wood is untreated like the inside of the cabinet.
7. If you want the tank strictly for freshwater ask if it has ever been used as a saltwater aquarium. This wouldn't be a good purchase if it had.
8. Bring enough people to move it, especially if it's big. Communicate with the seller. Some are willing to help. Some can't or won't.
9. Give yourself enough time. If you're rushed, this is when mistakes and judgement errors can happen.
10. Ask the seller about their interest in fish. Often the things they tell you will either settle your apprehensions or clue you in that something is not right.
11. Even if the tank looks clean (if you buy it dry), clean it again. You never know if whoever broke down the aquarium used a chemical cleaner and you don't want to start out with tainted water and not know it until you add fish.
I'm so thrilled with the purchases I've made off of craigslist. People have been pretty honest with me and I've gotten deals that I could have never afforded new. I hope your experience turns out as good as mine have been.