I just looked at the ebay post for the 20g long and Oscar. They are no longer selling the tank with the fish, they have a note on the bottom of the page saying that due to concerns of so many people the fish and tank must go separately.
At least they came around. That craigslist ad is brutal...I had two Oscars in a 55g, but as soon as I read more on this website I learned just how bad that was. I brought them to the LFS the next day, I couldn't stand to think of them being cramped in there. Just like someone else in this post mentioned, I got them from Petsmart. They told me you need 1 gallon per inch of fish and that I could have 5 Oscars give or take in a 55g. Luckily I only bought 2, imagine if I had gotten all 5! The two I had were about 8" when I brought them to a LFS.
The major reason for that is lack of knowledge. Thats what makes this website so great, I stumbled across this site by googling Oscar or cichlid, hopefully more people will stumble across this or other informational sites.