SKYSpy . .
I have to agree with davhamm. I have bought and sold 3 Sky RLs with GMPP turbo and CAI upgrades. With almost all interested buyers, nearly all were interested in low mileage, good body and interior condition, maintenance records and wheel / tire condition. Since it was considered "factory", the GMPP upgrades were a plus. Everything else, like Solo cat-back, DDM braces, etc basically were of no interest or a detriment to sale.
I know that you invested time and $$$ on customizing, but as davhamm points out, it will limit the number of people interested in buying. To command a higher selling price, the Sky may be on the market longer.
If you are willing to sell at a lower price point (say 15 to 16K), I think that you could sell it fairly quickly.
I used,Autotrader and CarGurus to get the Skys sales exposure. CraigsList was a bust. You may even find eBay a good way to sell. They get a lot of traffic. All 3 of my Skys were sold to guys who were hundreds of miles away.
Wishing you well and a high price on your sale of your Sky.