I have had fun making and using refractors with D&G and Istar lens. The resale value is usually less than the replacement cost of the lens and focuser.
I guess the market for this sort of thing is just really small.
This has been my experience also. But when you think of it, the cost of an objective lens with any pedigree is typically higher than the cost of a similar size mass- produced scope.
Istar achromats, 6" f5 is shown at $700.00. Their 5" f8 is $525.00. You can buy complete scopes with mounts at those prices and you get some comfort in a manufacturer's warranty.
If you build a scope around a lens with no pedigree, ( surplus or other unknown manufacture) it has no resale value at all. I have sold a couple of SS lenses and essentially scrapped the tube assembly I made. If you can get 75% of the last sale price for the objective, you have done well. If you spent significant funds on materials and machining custom parts, it is very unlikely that you can recoup any of it.
You have a better chance of selling a high-end piece of machining art, (think polished brass perfection), than you do of selling a well built optically perfect telescope. I am a tinkerer. Not an artist. But I am having fun.