Chemicals, (aka "mechanic in a can"), rarely solve problems. They aren't going to repair a mechanical problem. Slippage in a transmission is almost always caused by worn clutch plates, and it needs to be rebuilt to solve that. If you really want to buy a car with problems, get a quote for a worst-case cost from a transmission specialty shop, then deduct that amount from the value of the car to determine what it's worth. If this would be an easy fix, the current owner would have done it before selling the car or instead of selling it.