That's for me to know and you to find out
You wouldn't expect a real trader to tell you how he works out what price they'd accept from you, would you?
All you need to know from a players point of view it works the same way as described by the DMII guide / handbook / walkthroughs etc etc (i.e. place a sum of money on the table that's less than the advertised price and you may or may not get a deal).
All you need to know from a dungeon designers point of view is that there's a new parameter for SHOP_STOCK, which is the absolute minimum acceptable value for the trader (which they're very unlikely to accept, but theoretically might); they'll do the rest.