The advantage of buying a new stairlift is that you know every part is new and the stairlift has been fully tested by the manufacturer before you receive it. This substantially reduces the risk of failure and should mean that you won't have to replace any of the parts for a long time.
In addition, in the unlikely case that it does have a problem, it will be covered by the manufacturer's warranty. So you can get it repaired or replaced for free.
The only disadvantage of buying a new stairlift, is the price. Like with any electro mechanical device, you will pay more for a new stairlift.