If i remember correctly I recall seeing one of the mods saying if you buy a voucher from somebody, and the person who sold it to you charges back, you will lose the amount of the voucher.
Now to me, it's normally well if you deal with somebody un-reputable you run the risk of that, but tbh that shouldn't be how it is.
There should be a minimum requirement to sell vouchers, such as;
Certain time frame of being a member. 1-3 months
Certain amount of reputation
or even a post count rule(although I think the 100 PC for acc rule is meh)