BuyCraft is telling me that x is not a valid Minecraft account, how can I fix this?
In order to disable the account checking BuyCraft must think your server is in offline mode. Prior to version 0.3 the plugin didn't auto-configure this. In newer versions it shouldn't happen. You can try signing up for BuyCraft again and selecting "offline-mode" in the wizard.
How will this impact my server speed?
All I can give you is the details. There are three tasks which run on intervals:
CommandDeleteTask (runs every second; performs one outgoing request if commands have been run)
CommandExecuteTask (runs every second; will execute commands if any are waiting)
PendingPlayerCheckerTask (run interval is set by BuyCraft.net and is every 2.25 seconds; performs one outgoing request) (the request is sent async as of 0.7)