Alex Ferguson was able to sell players and replace them with proven quality by virtue of being the biggest/richest team in the land. Shouldn't need to be pointed out either that his United teams were top quality across the board and so less reliant on individuals, so it had less of an impact. Anyway, were Stam & Beckham their best players when they were sold? Not for me..
Arsenal right now aren't really comparable as they have shipped out expensive wasters/poor buys that the manager has wanted to sell, you could compare to Poch with Adebayor and that cabal of players in 14/15.
When would be the right time to sell your best players? I don't think there is a right time, more a case of least-worse time, and that time would be when you have a strong team across the board and confidence in your buying department to replace them well (similar to the SAF example above) the closest we have got to that was under Poch - but as we saw post WHL our buying has been shocking so the liklihood that a different approach to team building (wrt to selling our best players) would have seen an improvement is fanciful and would likely have just expedited our drop out of the top 4.
Right now we need to cling on to what little quality we have in the hope that our summer signings supplement those players and help make the team stronger as a unit