Upload & Sell: Off
May I start with a small point for clarity? As you likely know, there are at least two APO-Rodagon-D 75mm lenses. One has a maximum aperture of f/4, and is optimized for 0.8x through 1.2x (I'll call it the "ARD 75 1x"). The other has a maximum aperture of f/4.5, and is optimized for 1.2x through 2.5x (I'd call it the "ARD 75 2x"). (There also exist both fixed and variable aperture versions of the ARD 75 1x, the fixed aperture version reportedly being a bit better than the variable aperture version. Whether or not a similar variation occurs in the 2x ARD, I don't know.)
I have and use both of these lenses (variable aperture versions) for macro work, but have never tried them for digitizing slides. I can say that both are very good--far better, for example, than my EL Nikkor 50mm f/2.8 N enlarging lens. FYI, I also have a Printing Nikkor 105mm (which, like the 75mm ARD 1x, is optimized for 1:1) that seems better still. But the PN 105 is hard to find--and requires either an even harder-to-find mount, or a custom made one. The 75mm ARD 1X, on the other hand, is easy to obtain and mount.
These lenses have been discussed several times at http://photomacrography.net/forum/ , though for macro use, not copy work. Also, Mark Goodman has posted tests of both these lenses at http://coinimaging.com/Lens_tests.html for coin photography. These discussions are informative, but might or might not help you decide if the lens will improve your slide copying.
Your report that your efforts with an enlarging lens have produced better results than your Nikon Coolscan V is very interesting. This being the case, I suspect that the ARD 75 1x may be a worthwhile upgrade to you; if you have especially high standards, perhaps even the PN 105 might be worthwhile. While the law of diminishing returns certainly applies, personal needs and judgement dictate where the cutoff point occurs in the cost/benefit curve.
Thanks for raising an interesting point. I use a Nikon Coolscan IV for digitizing my slides. But since I already have the optics, you're making me wonder if I'd get better results with my Printing Nikkor 105mm. Certainly, shooting with the PN 105 would be quicker, and would more easily facilitate focus stacking for curved slides, and modest HDR for potentially improved dynamic range. Whether this would make a real-world improvement is open to question, but the experiments should probably be conducted.
Given that these optics are available in open auctions, the cost to purchase, evaluate, and then sell a lens that does not suit your work seems limitable to the transactional costs of buying and selling, plus the increment between your price and the price of the next highest bidder. With a bit of care, doesn't this seem like a reasonable cost for finding out if the lenses offer justifiable improvement for you?