This is a myth. Levy never sells players unless they start creating and / or are nearing the final two years of their contracts and refusing to sign an extension. That is true of the only players that Spurs have sold that they wanted to keep since 2006 - Carrick, Berbatov, Keane, Modric and Bale. No other players have left against the club's will.
The last of those players to leave - Bale - did so four years ago, since when Spurs have been steadily improving. They can't match the wages of the wealthiest clubs but, in most other respects right now, they are as good as any club in England. So the best players aren't in such a rush to leave any more, especially since they have complete faith in the manager (who has dramatically improved the vast majority of them). But even if they were to be tempted by the money on offer from elsewhere, Spurs have almost all of them tied to long contracts. Levy can pretty much name his price such that any deal simply wouldn't be worth it to any other club (and we can be quite certain that that is exactly what he would do in such circumstances).