Tom, I had a quick look at your listings and noted a couple of things:

1. You use a generic condition description - "In "Very Good +" condition cosmetically and in "Excellent +" condition optically."

2. You have a templated blurb that describes how you strip and service all lenses.

I think if you are not noting any item specific faults for individual lenses, you are opening yourself up to buyers being unhappy if there is anything at all wrong with the lens in question.

The old sales adage of Underpromising and Overdelivering is something of a truism.

Also, photographers are picky as they are buying a lens to shoot pictures and if that lens is soft in the corners or stiff or loose they will understandably be upset having read the lengths you go to in service.

A lot of buyers will also not understand that although you clean and remove fungus etc, that the fungal growth will have destroyed coatings on the intermediate elements and that's probably where the edge softening is coming from.

If you have a full frame DSLR you could always try shooting a lens chart for each lens though I don't think that would be a perfect solution by any means.

Probably, you'd be better off shooting a sample pic or two on film then scanning a print as illustration for each lens while noting any mechanical issues.

You've found a very tricky area to sell in I'm afraid.