David, I think most of the builders have a low percentage of 1st time buyers that decide the Speedster Replica is not what they had envisoned. These are basic sports cars, even in the top price range of builders. So, there are people that bail on their first purchase after they realize these cars require upkeep, TLC, and are not a new Miata... Vintage builds more cars, and so it stands to reason that you will see a few more for sale. The other used Vintage market consist of a few owners that have decided they want to upgrade to another builder.
For the few VS cars that you see for sale, there are hundreds that have been owned for years, and are being driven as much as any other brand.
I had a Vintage for 3+ years. It was a great buy, and a solid Speedster. It is owned now by another member that drives it and enjoys it as much as I did. That car has at least 35,000 miles on it by now.........
Just a little info to think about. If you are wanting to get into a good Speedster today for a decent price, then Vintage makes for a good choice.