Okay don't take this the wrong way-- I'm being a bit silly but ask a few questions to get a profile of the seller. So basically in my area, most tanks are sold by younger dudes perhaps students and their equipment, although brand name, is beat up and crusty. The other main seller I meet are the parents of multiple kids under 10.... This is the best seller because they clean the tank before storing it in The garage and they have a dream set up but haven't touched it in 3 years. Back to school clothes and what have you- time to sell and just make an offer but take it all.
The golden listing, once in a blue moon, is the oceanographer who is selling one of the tanks in his office. He charges a bridge toll for the whole tank. Man I missed that ad once, ill never forget