I didn't make it clear but I mean this only applies if it turns out their best player is of Michu quality/potential price tag. The £20 million or so that Swansea could get for Michu if he keeps this up would be faaaar more valuable to them than the £25/30 million that Arsenal/Liverpool got for selling their best players.
That, frankly, is utter garbage.
Swansea (or clubs of similar stature) selling their best players is completely different to Arsenal or Liverpool selling their best players.
For teams like Swansea, it's significantly easier to sell your best player but improve your team.