Hello; I bought a used tank many years ago that was dirty. After it was cleaned i discovered that all the surfaces of the glass were scratched. The scratches were too notable for the tank to be used as a display tank. My lesson from this is to not consider a dirty tank that is not free.
As far as the chipped glass, as far as I know the only way to find out is to use a tank and find out the hard way. Some hold while others do not. I have had a tank fail that had no visible flaws. If the tank can be set where a major leak will not do floor and other such damage, then losing $50 may not be a big deal if you have the money to lose.
Good luck
