I want to open a Minecraft BungeeCord network and I'm looking for a method to connect Buycraft (X) to all servers in order to buy a rank that it has on all servers.
You can forget that. You must be at least 18 years old to use at least PayPal as a payment method. For every other service provider you have to be self-employed.
In order to connect Buycraft / Tebex with all servers, there's an interface / plugin especially for bungeecord. Find out from Buycraft, everything is described there. (English)
