Hi and welcome! I haven't seen a thread on this, but I can tell you about my own experience (I'm still getting my tank set up).
I bought my 90g aquarium and stand w/ canopy used, which I think was a great deal. It came with a heater, sump, powerhead, battery back-up system, and HOB filters. While I think the tank and stand were completely worth it, I've chosen to ditch the HOB filters and replace the heater (the guy had it for years, and most go bad within a year or so), the powerhead (the 90g I bought actually needed more flow than the one powerhead would provide), and all of the hoses connecting the sump to the tank (they were filthy). I've also learned that the sump that came with the system is a little on the small side for a 90g, and so I've needed to look at supplementing it with a refugium, and will probably upgrade it down the road. In addition to being a bit on the small side, it is also an odd set up for a sump, and after some research, think the model was purchased off of amazon. I've needed to do some plumbing on it, etc., to get it to work for me. I also ended up replacing the lights because I wanted a set up that would be good for coral, and have invested in a protein skimmer, auto top-off unit, and RO/DI unit. Overall, I've now put well over 3k into a set up that I thought I'd pay a lot less for ($400 for the initial tank that was advertised as having everything I'd need). I'm not unhappy about it, I made the choice to upgrade a lot of equipment, and still think I got a pretty good deal for just the stand, canopy, and tank for $400. If I were you, I'd really figure out beforehand what you want to do with the tank - corals? If so, what type? LPS and SPS will need higher-end lights to survive. What kind of fish do you want? If you want angels, triggers, tangs, you'll need to be looking at a pretty good sized tank. If someone is advertising hang-on back filters or canister filters, that's not a good deal - you'll want to ditch those and go with a sump instead. Most of the filtering will be done in your live rock, and you'll want to make sure to get a protein skimmer and RO/DI units. If it comes with a protein skimmer, find out the brand - there are a lot of crappy brands out there that won't do you any good - you are always welcome to ask here what people think! I don't think it matters so much if it comes with a sump or not, you can make your own out of a small aquarium or even a plastic bin from Target/Walmart/etc pretty easily. If the tank isn't reef-ready (drilled), then it would be nice if it comes with an overflow, but again, you can buy one of these for around $100-150 depending on size. Check out the basic equipment list under the "articles" section of this site, it has great info on essentials you need to get started. Bottom line, from my experience, buying the tank used is a good deal, but investing in new equipment for the tank is probably the way to go. I've also seen a lot of higher-end listings on CL for tanks that come with fish, corals, etc. In the beginning I didn't even bother to look at a full set up if it was listed over $1000, but now that I've got my own set-up, I might have saved some money had I gone with an already up-and-running tank from CL. Good luck & keep us posted!
