Did you try the demo of what you bought before? If the answer is no, then you aren't entitled to a refund.
Did the item have a demo? If the answer is no, and you still went through with the transaction, you aren't entitled to a refund.
Did you buy something two times, by mistake? If the answer is yes, you should polity settle this with the seller as most stores offer a refund if the items come with copy permission.
The only reason I would see for a refund is being scammed, but since you're not buying from LL itself, it isn't their responsibility to give you a refund.
I admit, I was scammed, in the past, having bought something that perhaps was marketed to seem what it's not or just not having the right permissions. At first I was pissed off, but now I'm not because now I know what to look for when shopping on the market.
If in doubt, just don't buy it is the best policy for shoping in SL.
PS: this isn't RL and people should stop comparing the two.