I'm cheap
A fair price would be $1/gallon plus about half the online price for used equipment.
But, I'm cheap, the last few tanks I've gotten were almost new tanks with nice cabinets plus all equipment (2 eheim canisters per tank) even food and fish for the cost of the glass new. That was $250 for a 58 gallon and $500 for a 105 gallon. I also paid $250 for a 58 gallon on an antique cherry stand with two Fluval filters, but I over paid on that one, the equipment was a mess.
So, if the equipment is rather standard, I'd offer $200 and I'd go as high as $250 which is $100 for the tank and $150 for the stand assuming it is nice. If it is a friend, then play fair and don't low-ball them. If the stand is ugly pine or homemade then I wouldn't pay more than $50 for the stand. If it is nice den quality oak or cherry cabinet and has top of the line equipment, then more, maybe $300 to $400.
I got a 30 gallon for $5 just the glass, and a 55 gallon for $25, and a 55 gallon plus stand and powerheads and internal filters and wet/dry for $55. But those were all garage sale bargains!
