"Force Sell"

Is there any way to force a shop to sell an item to a unit? (under normal conditions; the unit will have enough gold and will be in range)

I dont think so, but you could make an exact replica of it though.
Remove the exact gold amount (via the GetItemCost function)
Add the item (via the Add item to hero)
Remove stock by 1 (via the Neutral Building functions)

Try ordering a unit to <item> an itemshop or something :s

It's GetUnitGoldCost which is in common.ai (and works for items too). There probably are several triggered functions for this though, eg on wc3jass.com