A used/repaired transmission may be way cheaper than a FULL rebuild. However, the former will not eliminate the problem in the long-term. There is always an element of risk when purchasing a used transmission or having the transmission repaired.
A used transmission will work for a while after the installation. However, it may contain interior damage to some of the gear, seals, valves, or may carry manufacturer defects. It may be initially cheaper to buy a used transmission, which is likely to come with damage or flaws. However you will likely end up having to spend much more at a later time, when you will realize that you’ll need to opt for a FULL rebuilt.