"Flipping" Petco tanks on Craigslist will not earn you more than an hour, maybe two of minimum wage in profit. (ie. the amount of work you'll probably put in to buy/sell them in the first place)
I bought some tanks at the local $/gallon sale today. The 40s are just awesome, if you have good solid stands for them. Don't neglect the 20H/29/55s for ripariums, either.