I trust maplestoryer, the mesos company, i have purchased there atleast 3-4 times, and it's trustful, they may take up to 8 hours to deliver though, but they are trustful, but the downside is the time they take, i had to wait atleast 4 hours to get 600million mesos.
Also if you get scammed, you can report them to the billing company, and they will get a refund on your money or something. (other sites)