An Aftermarket part is a part that is not made by the manufacturer. Aftermarket parts are made by a 3rd party company independent from the watch manufacturer.
There are two common reasons a buyer may choose to buy a Rolex that has been customized with an aftermarket part or parts. First, it allows the buyer to pay less for the Rolex, when all other factors remain the same for age and condition. Second, it allows a buyer to get something that Rolex does not offer.
For example, a buyer wants a ladies stainless steel Rolex Datejust from the 1990’s with a diamond bezel. However, Rolex did not permit or produce diamond bezels for a ladies stainless steel Datejust produced in the 1990’s or earlier. Choosing aftermarket would allow the buyer to have a diamond bezel on the watch.
The value of a Rolex is based on, but not limited to 3 primary factors. AGE, CONDITION, and ORIGINALITY (OEM). “OEM” means “Original Equipment Manufacturer”. The more original (OEM) a Rolex is, the more value it holds, and the more it will cost. For an example, If comparing a Rolex that currently has an aftermarket band to a Rolex that has an original Rolex band… and the original Rolex band was in perfect condition… then the value of the Rolex with the original band would be more than if it had an aftermarket band. If the Rolex had an original Rolex band… and the original Rolex band was in poor condition… then the value of the Rolex may be the same or less than it having an aftermarket band.