If it is a voucher from a cruise line that was given out for some reason like compensation for a missed cruise, etc, then it has nothing to do with the cruise lines or agents.
It would be like if I took an airline voucher on an overbooked flight for $400 and then sold it on eBay for $250. It is transferrable, so I see no problem with that.
If travel agents have a problem with people selling legitimate vouchers like that then I think they doth protest too much.
I have personally never heard of other kinds of cruise vouchers that are legitimate but if you want to PM me you certainly can.