"Every club will want Ross as he is going to be an outstanding player," said Moyes. "I'm worried it will be a difficulty for us in the future.
"If you're going to come and try to take our better players we don't sell cheap, that's for sure."
The young midfielder's talent has been likened to that of Manchester United's former Everton teenager Wayne Rooney, who left Goodison Park in 2004.
"Can we carry someone like Ross now? Of course we can," Moyes continued. "Maybe when we had Wayne I didn't think we could hang onto someone of that talent, at that period."
Barkley has already made three appearances at under-21 level for England.