I bought a used 29g biocube off CL
and had a terrible time with it. When I got there to get it it was holding water fine. Took it home and it still held water. Only when I added the sand and liverock did it start leaking. Guess it couldnt take the added weight or something.
Just be very careful and check it out for any issues. If it's empty check inside the at the seals.
Hope that helps and good luck!