If you ever buy a used aquarium, here is what usually works very well for me.
The first thing I do after buying a used aquarium is inspect the seam for leaks. Then give it a good scrub with just a soft sponge/paper towel/wash cloth.
The next step is to wipe it down dry.
After that, I usually do a vinegar scrub.
Dry that off afterwards
Next, do a saltwater scrub.
Dry it off once more.
After all of this is done, rinse the tank thoroughly.
You should then proceed to let it air dry.
There you go! Your tank is clean and ready for you and your fish to enjoy!