I don't have any experience buying mountain bikes, but I have found that my patience plus extra small needs (~43 cm) have allowed me to get great deals on used bikes.

I own two titanium road bikes from top custom builders. The first I paid about 1/3 the price of new for a perfect bike with only 500 miles. The second I paid 1/3 the price of a new frame for a complete bike (although some of the components were worn and needed upgrading). It takes patience but when these tiny gems are for sale there aren't a lot of bidders. Buying on eBay doesn't allow me to test ride, but I know what geometry will fit and what won't work so I don't worry.

I wondered what I'd do if I won Fat Cyclist's last fund raising contest - the prize was your choice of any Ibis bike. No models came in a small enough size for me. They make great bikes!