If I were looking to sell off my 70 I would definately offer it. But, it's become quite valuable to myself as a
frag tank (which is something I didn't expect to be finding of use after only having my tank setup for a little over a year) thus I can't sell it off yet. That and it works well to keep my psycho yellow tang out of my display tank. It literally hates any tang anywhere near it. When I got my powder brown I had a net between the two in the 70 and the yellow would literally attack the netting between them lol. It was crazy.
If you do upgrade... You might as well go big as you will only regret it later otherwise. I went from my 70 to a 125... And I wish I would have even took one step further... But, didn't feel that I would be able to afford the maintanance on it... But, it turned out to be cheaper to run than my 70 gallon alone was. The only thing that is expensive is finding the equipment to set it up (skimmer, pumps, and lighting)...
Jason