Yea, you can find some really great deals on CL, Facebook, Offer Up, and Letgo! It's one of the best things about the hobby.
I hate the people who overprice and charge way more than a new tank. It seems all they are after is the money.
As mentioned a good starting point would be a 75 gallon, but the saying is true bigger the better. A 120 gallon is perfect.
I saw on CL someone selling an old dirty 75g with an emperor 400, lids, heater, gravel and a few cheesy decorations for $700 lol! Again, I could spend about that or less on a NEW 125g. The gravel and decorations must have cost this person hundreds of dollars or tanks appreciate with age lol.