To me that shows the predicament Everton currently find themselves. They sold Fellaini for a big fee and replaced him with players who cost less. That's where Everton are - big offers for players will force their hand, even if they dont want to sell.
Barkley looks a very good prospect - but Everton arent a one man team and he could arguably be replaced if a club was to come in and offer silly money based on the player he might become. That might give Martinez money to go out and sign a number of players - or (for example) keep Lukaku at the club.
At the end of the day, at most PL clubs the books have to be balanced and Everton seem to accept that. Lescott is a good example - the bid was just too much to turn down.
As I said, he looks a very good prospect, but there's a difference between that and really becoming a top player. He could be a Bale, or he could be a Jeffers - depending on luck with injuries and other factors so Everton might consider any massive bids that come in and could help them move the club forward long term.
As you say, he may be happy to stay and if so, fair play to him. All this just sounds like what Glaston was on about last year with Bale.