I'm curious as to why when you looked at the tank, saw it was odd (you admitted this, and said you "just figured..."), you didn't ask more questions, or start running your hand along the bottom, pushing a little bit.. Or simply walk away from it, since it didn't look right to begin with.
You knew it looked "off", and bought it anyway. Unfortunately, I think at least 50% of the blame lies with you. The sellers were honest in stating it was "repaired". That right there would tell me to run.