When trying to sell my car fast I spend an inordinate amount of time on the transmission. The car is less likely to move fast and sell fast if my transmission is on the brink of dying. Of course there are different things that may be considered depending on whether the car is a manual or an automatic. If the car is an automatic you have to consider transmission fluid more than you would need to if the car is a manual. On the other hand if the car is a manual you will have to worry about the clutch. If the clutch is slipping usually denoted by the engine working harder without increasing speed then you will need to have it looked at by a professional.
If I want to sell my car fast I ensure that all of the details about the car as polished as possible. If the car is an automatic and the gear changes are not smooth then it can be an easy fix. It may just be that you need to top up the fluids in the gear box. If you top up the fluids and the problem still persists then you are going to have to have it inspected. It may be indicative of a bigger problem. There are too many moving parts in the transmission to take a chance with it. It only takes a small problem to eventuate in a larger problem which in turn can lead to having to replace the whole transmission. This is not a cheap thing to do and an expense that the buyer should not have to incur.
Of course the first step in ensuring the transmission is in good working order is to make sure you take care of it while you are driving the car. It helps me to sell my car fast if I don't have to go out and spend a small fortune on getting it repaired. If your transmission is in poor condition and your buyer does not have an enjoyable experience driving the car then you may have lost your a potential buyer. A lot of problems that appear to be engine problems may in fact be relevant to a transmission problem. The first step is ensuring the transmission is topped up with fluid.