What's the typical markup on a used Porsche from a dealer? (how low can they go?)

Is eBay better than my local dealer? By how much?

(fyi my target vehicle is a 911 Cabriolet, manual, dark blue or slate,
2001 or later)

I imagine that there are too many variables here to allow someone to
give you a comprehensive answer, but perhaps this will be of help:

kbb.com (Kelley Blue Book) can provide you with general information on
what you can expect to get for a trade-in and to buy from private
parties and dealers.

You can search completed eBay listings to see what these vehicles
usually sell for. Of course, part of eBay's appeal is the opportunity
to get a great deal.

Have a good day.

good point. i've been using kbb, just not sure how reliable their info
is. but thanks anyway!