Overvoltage doesn’t actually burn motors so the seller is not correct. If rpm is too high then eddy current heating can burn motor but i have never seen this happen abruptly. Then more likely that the esc couldn’t control at that high switching rate. 500kV 50v is 25000 rpm, with 6 poles —> 3 pole pairs that’s already 75000 erpm which not all esc can control at. (is this motor really only 6 poles, not 6 pole pairs?)
You can burn motors with
-too much current: some escs lack basic current control or current setting was too high or
-too low efficiency: esc cannot drive motor correctly by it’s design or parameters in settings were wrong for the motor.
There are some basic tests you can do to find root cause. One is to have motor connected to esc without power on, turn shaft by hand. If there’s increased force compared to motor not connected to esc then esc is failed to a short.
To measure turning force with two phases connected to each other does not prove that winding is ok, it shows that there is still connection and magnets are not totally cooked but not if the winding is ok.
I guess a broken esc is the most likely problem. Could also be a short in the windings causing an isolation problem and then possibly also the esc to break.
To verify this:If you have a big short circuit then you can find it with a continuity test with a multimeter. Set continuity measurement, connect one test lead to a phase and probe axle, bearing, motor shell with the other. Poke hard enough to scratch to ensure a connection.
If it beeps or shows connection then winding is shorted. If so, don’t try a new esc as driving a motor with a shorted winding could break esc, motor will need a new winding.
If there is only a tiny short in the windings then unfortunately you need a dedicated isolation tester to find it. I’ve had motor with this kind of issue and my esc wouldn’t drive it at all, couldn’t find the issue with multimeter but tested with an isolation tester the motor was indeed shorted but with some resistance in the short circuit.
It’s not hard to open these motors so this is a quick way of seeing if there are any obvious problems with the winding.
Good luck! If you find a root cause then please report it here, it will help others.