I attempted to buy 3 Karmic Retributions (it costs 5 gold and 1000 unbound magic) at once from the vendor in Ember Bay, since the description says the effect stacks up to three times. And the vendor did indeed take 15 gold and 3000 unbound magic, but I was given only one stack of the effect. Apparently each level of the effect costs a different amount (level 2 is 10 gold and 5000 unbound magic it seems), but the vendor will happily steal gold and unbound magic if you try to buy more than one at a time.
I don’t care about the 10 gold lost so much, but the 2000 unbound magic lost is pretty brutal as I do not care to PvE that much.