I only have 5,600 miles on my GTS, so I'm not in the market for new tires, so please take this as "friendly" advice from a fellow 718 owner. I happened to be on Tire Rack shopping prices for new tires for our X5. I never buy from them, as our local independent (Radial Tire Company in Maryland) offers equal or better pricing for PCA nd BMWCCA members that includes mounting/balancing along with free rotation, flat fixing etc, for the life of the tires. So for me - and perhaps most Porsche owners - buying a used tire, even with only 1k miles, and going through the hassle of picking them up, finding a quality shop to mount and balance them, and possibly not have the manufacturer road hazard warranty, the savings vs. brand new would have to be relatively significant.
I certainly don't fault you for asking $375 each vs. Tire Rack's $420, and hope you are successful in finding a local buyer who may have just encountered a spike strip and is desperate to get his car back on the road. But if you put yourself in a prospective buyers shoes, would you really go through all of the hassle of buying a used tire to save 10%?
Again, not in the market myself and not negotiating, and hope you are able to effect a reasonable sale. FWIW, I still have a new, in the box high end basketball post and backboard that I decided not to install 15 years ago that I could have sold to a neighbor for 50% of the $1,400 I paid for it, but I thought that was too much of a discount. Shame on me.