For me, it would definitely hurt the resale value. By a lot? I guess that would depend on the specific sharpening job, but I would never consider buying a knife that hadn't been "used", but had been sharpened, for its full retail price.
To me, if a knife does not have the factory edge, it is quite used indeed. If someone has sharpened a knife, all they've done is scraped away some of the blade to suit them better, and their sharpening job might not suit me at all. Even if it's pretty well sharpened, it doesn't really do me any service whatsoever - it's more like they've 'worn it in' than anything else. I use all of my knives, so I'm fine with buying used or pre-sharpened knives, but I'd want folks to tell it like it is. Given the choice, I'd kind of rather get a knife with a dull factory edge that had seen some use, etc. than get a knife that has had a big sharpening job done to it already. The knife with the dull-ish factory edge is kind of like a clean slate that I can now put my own edge on.
A less important thought that comes to mind is that Sharpening a knife does, eventually, wear down the blade - a knife that's already been sharpened heavily by the previous owner (say, a drastic reprofiling or whatnot) has that much less life left in the blade.
As a seller, when I buy a knife, sharpen it, and then decide to sell it (whether I've used it or not), I fully disclose that it's been used and sharpened and treat it the same way I'd treat any used knife - I generally look to get a fair price consistent with a Used knife, not a New one. :thumbup:
So, in short, I'd buy a knife that had been sharpened by the previous owner, but I would consider it 'used'. I'd sell a knife that I had sharpened, but I'd sell it as a 'used' knife. My jimmies would get a little rustled if the seller expected me to pay full price for their used knife.
