Both Dometic and Norcold offered the ice maker option to the OEM and the OEM than offered it to consumers. A very popular option means good news, nearly all the components are available AND the refer manufacturers provided a means in nearly all their refers for easy after factory install.
Check the back wall of the freezer. There should be 4 mounting holes. On the right side or ceiling there will be a hole or a couple holes for the water tube, and electric cable.
You can find the ice maker on Amazon, along with all the wiring, the tubing, and valve kit. These items have been available for DIY'ers for quite a while, and haven't changed much over the years. So tell a new owner to not pay too much. For instance, the ice maker itself is a $60-$70 item on Amazon, but they also carry the exact same model from a different supplier for $300.
With the refer outside the RV, that makes installation easier, under an hour, but if you or they can't do it the job, you have to pay a mobile tech trip charge and hourly rate.
I would guesstimate a potential buyer would be charged $300 for the entire job by a reputable shop, including parts. If they bought the parts themselves, and hired a mobile tech, they'd save some money.
Doubt that a potential buyer cares all that much about the icemaker though. They're buying the refer because theirs isn't cooling.