More pacifically - is there a market for a used Extremely Popular make/model of pistol that's had the slide milled for a MRDS - but the MRDS wouldn't be sold with the pistol?
I suppose, like anything, there's a market for everything if the price is right...
So long as the slide has been milled for a popular optic footprint (RMR, Shield & DPP), I think there is definitely a market. Even more so if you sell the optic with the gun. Also, so long as the gun has been milled for an optic platform that is right for the gun size. Example....a G48 milled for a Shield footprint, not an RMR footprint. YKWIM?
For some guns, you can replace the sight with a filler plate if you want to sell the gun without the sight. AllenM milled my G23 for an RMR but I've since moved that RMR to a different gun. I picked up a nice RMR filler plate off of Amazon that let's me run the gun with irons now. I think that the plate was less than $20
Selling a gun with the red dot attached either inflates the price out of many budgets or loses money spent on the red dot. The plate is a nice option in that it keeps the gun cheaper but offers the new buyer the option of running a red dot if they want.