Elite Dangerous ship sellers have an interesting marketing strategy:
"No try before buy. Give us $50M and we give you a ship. You're not even allowed to look at it before hand!"
Probably the best reason not to allow this is you aren't actually buying a complete ship when you buy one. You're getting a hull and the bits to get it to a place that can outfit it (if it can't be done locally), and really, the stock parts aren't at all representative of the capabilities of a ship in the slightest.
But it doesn't have to be a vanilla ship. Player should be able to specify the loadout he wants to try out. From an in-universe standpoint, it can be a VR simulation ran by shipyards where they can put you inside any ship you want with any loadout possible. It could actually be a paid service, costing say 1% of the actual ship's price, which will let you either fly around a couple of "generic" star systems to test superscruise and jump range or engage several simulated enemies in a dogfight to see how ship handles in combat. INB4 Yo dawg I heard you like space simulators
As an alternative, ship/loadout selection should be added to the tutorial after it was completed in the default sidewinder.