DIY!
I have a 180 gallon built into a wall cabinet and I probably have less than $1000 total into it as far as tank, cabinet and all equipment. I made the cabinet and all filtration. It is a softy tank though. I'm using a $50 harbour freight pump that pumps water up to 2 surge tanks mounted over the display. The surge maximizes the flow. I built a T5 4' x 4 lamp that mounts over the surge tanks. I have two ATS's and macro algae in the surge tanks. I also use sunsun wavemakers. One is a dual 3k+ gal/hr that has been working continuously for 5 years now and it was $30. First lights in the display were converted warehouse metal hailides that I got for $25. They were 400w and kind of overkill so I switched to some chineese black boxes that have been going for 4 years. One I even spilled a bucket of saltwater on and its still going. I also built an internal skimmer that sits in the corner. I don't know why they are sooo $$$$. It's just a tube with bubbles.
Man, OP, would a great deal. I just drove 3-ish hours each way to pick up a used tank and stand (IM 50 AIO) that had only been used 1 year (manufacture date mid-late 2019) for $400- came with factory return pump (I already have a spare Sicce return pump), caddy, and way more filter socks that I would ever use.
I agree that small discount is not worth the lack of warranty. I see expensive things on sale all the time for like $30-$40 off sticker price, but at the price, it's probably better to just buy brand new and not worry.
If anyone wants to sell a couple of Xr15's G5, I'm all ears
Non life support related gear like lights, powerheads, and skimmers are typically safe to buy used.
Essentials like a heater or return pump is where I would draw the line unless I have a back up in the case of the return.