I've seen groups such as "Legit Seller" and stuff that people list prices for, for various games. How do they do this? My son wants to purchase cheaper credits, but I don't feel comfortable buying them for him until I do a bit more research and understand the whole process behind it.

Example: The person my son wants to buy from says they need login info, proof of payment (send to a PayPal account), how many credits they want, and they get the credits on the account. I don't think it's a simple currency-rate exchange either. The guy is in the UK apparently and there are some people in the Philippines that do it as well?

I'd just like to know what's going on here and whether it's legit? I've been told that several of the people from his guild/team/group/etc. have used this person and they are legit, but I am curious how the heck it is possible to do that and how it's done.