Craigslist is amazing, I was going to buy a 70gal for $60, but I can't convince the lady that I need that, my 55gal to be turned into a vivarium, and stuff for my motorcycle....
I would think they would post it on the petco news site they have. They did it last August and you can sign up for their newsletter to get the special news when they have it. Last year it was Aug 27,07 to Sept 2. Soooo be on the look out. I haven't received an email about it yet so not sure if they will be doing it around the same time or not.
Ooh... I think I will probably have to wait until next year's sale, but that would sure be an ideal way to start my saltwater hobby. My original goal was a 90 gallon to start with. But a 75 wouldn't be terrible...