Different cases though surely? Welbeck is a player he probably would have loved to have used in his boring cunt ways, whereas he doesn't seem to rate Martial much at all. He was also the local hero compared to the lazy French lad. The fee was a lot lower too, and the whole point was to bring in a one legged Falcao in his place which was a mistake. Even with Ba, that was giving us depth at the last minute when we were thin and for probably a pittance fee so you can see why he pulled the plug on that too.
If Arsenal were the only club happy to pay 40m+ for him and he has very little use to the club, it's a bit silly to get precious over players you don't even like. Especially when the alternative is Arsenal getting players like Lacazette or Mbappe.
I think English clubs will end up selling more to each other now that they're the only ones with spare money to throw about. Nobody outside England is going to pay anywhere near 40m for Martial IMO.