That asking price is a deterrent regardless if you're open to offers. There is no shortage of pristine GTIs and its a buyers market. Right now you could get a newer CPO GTI with fewer miles for less. I was looking at a CPO 18' GTI DSG Autobahn w/39k miles for my wife at $17,900 and passed on that because she didn't like the color. She can because they are in abundance.
If you NEED over $17,500 for that car I'd suggest doing the following:
1. Aquire and post the CarFax in PDF format for folks to see
2. Elaborate on the status of wear items. Pics of rotors, pads and tires (if ANY of those aren't at 90% or greater life, replace them)
3. Close-ups of wear prone interior areas like the driver's seat bolster, foot wells etc. A seat with 34k miles driven by a primarily jeans wearing owner looks a lot different than one from a khakis wearing driver. Potential buyers will pay more for that.
4. Close-ups of paint in good lighting. Does it need $1k of correction to fix shoddy drive-through wash jobs? Has the rubber been baking in the sun? Any PPF or Ceramic and if so, who performed the work?
Point being, you need to post more data so folks can deduce that hey, maybe this particular specimen is worth an extra $1,500. Leave nothing to your word or KBB, neither are of any real value when dealing with educated buyers. Let the data talk.