Don't quote me on this (I am not a used car sales executive consultant and thus know nothing about cars) but I'm pretty sure the Stratstone Collection is what Stratstone dealers use to sell cars they receive in part exchange which are not part of that particular franchise.
For example, if you trade in a VW Passat at a Stratstone Volvo franchise, that Passat might then be retailer as a 'Stratstone Collection' vehicle. You'll probably find the approval process and warranty offering is not quite as cast iron as a conventional used car, but the prices are also far keener.