There is an enormous difference between a used and Remanufactured transmission.
A “remanufactured” transmission generally is from a manufacturing facility and carries a three-year warranty along with a mileage warranty between 50,000 and 100,000 miles. They will take a “bad” transmission, disassemble it and replace electronics, necessary soft & hard parts followed by a dyno test. This test ensures that the new transmission at that moment in time is operating without fault.
The “rebuilt” transmission depending on the shop will carry a 12 month/12,000 mile to a 24 month/24,000 mile warranty. Often you will be offered a longer warranty for an additional charge. A rebuilt transmission should be totally disassembled on a bench, inspected, cleaned and reassembled. The parts that are replaced are broken, damaged, worn, or cannot be reused. Soft parts such as gaskets, seals, o-rings, etc. are also replaced. The electronics may or may not be replaced depending on the inspection and the shop policies. There are almost always factory or aftermarket updates to transmissions. This means that when the manufacturer of the vehicle discovered a problem, or a part failure in the design of the transmission, they put out a “fix” which is called an update. Sometimes this is due to a known issue. The automotive aftermarket will also make improvements in design or functionality. Often those will be incorporated into a rebuilt transmission as well.
The used transmission is from a scrap yard and you have no idea how it was driven, if it was maintained, or the mileage. Now the scrap yard will generally tell you the mileage as it was entered into their computer when the car arrived. This used transmission will generally carry from 30 days to a 6 month warranty. Your transmission is removed and the used one is installed.
Now there are several reasons to choose a used, rebuilt or remanufactured transmission. First determine the value of the car including the mileage and condition. A good rule of thumb would be to determine your other options if you decide not to repair your car. Can you afford to buy another car? Can you get financing? Sometimes the “devil you know” is better than “the devil you don’t know.” That’s a really old saying. If you have taken care of the car and it doesn’t need hundreds of dollars in other repairs, replacing the transmission may be the best option.
The most expensive option will be a Remanufactured transmission and it will carry the longest warranty. A used transmission will be the cheapest and carry the least warranty. A rebuilt transmission will be priced and carry a warranty somewhere between remanufactured and used.
Ask yourself if you want to keep the car for several years or a few months. Often this answer will make the decision for you. And, of course, that question would be followed up with what you can afford. I would caution anyone about a used transmission because you really are rolling the dice. It may work fine but it may not even get you a month past the warranty you receive.