Quote from wowhead:
Run this script in the chat and then when you buy things make sure you talk to the Lovely Merchant and keep it targeted while buying!
Code:
/run local f=BuyMerchantItem BuyMerchantItem=function(s,c)if 37674==tonumber("0x"..(UnitGUID("target")or""):sub(7,10))then f(s)else f(s,c)end end
When you right-click the items you will purchase them without errors, yay for mini-addons!
Note: this is lost when you reload the interface, disconnect, logout and so forth. In short, don't worry, this will not stick and ruin the game, if problems occur just simple relog will fix it. 
Also for those interested in the technical aspect of the script, all it does it overrides the built in function that is used when you buy items from merchants. In short, when you target the Lovely Merchant NPC it will skip the quantity check but if you don't target it while buying it will use regular buying behavior. This being said I don't think buying in stacks (shift-clicking and buying a specific amount) works with these vendors, but I doubt it will be a issue.