There's good reason for buying a 'brand new' par
Rubber oxidises, cracks, perishes, wears out, loses its elasticity - probably more, so being in close proximity to heat from the motor.
If the original poster buys an old, used model, the motor mounts will probably be of the same condition, i.e. already on the way out.
If the motor mount is worn out, the motor will not sit square and true. The motor sits squint. The belt can then ride along the motor spindle, possibly falling off the end - but not so likely, as Electrolux had a tendency to weld a washer onto the end of the motor spindle to prevent that happening.
More likely, is the opposite to occur... the belt rides along the motor spindle towards the motor. The belt cuts through plastic, and possibly slicing through the rubber motor mount too. Usually, the soleplate also bears the scars of such an occurrence.
That happened to me, with a Dyson DC01.
A similar effect can also happened where the motor spindle wears down just through friction. For some reason Electrolux motor spindles seem to be made of a less robust metal.
That happened to a friend's parents' 500 Series, a customer's 500 Series machine, and me with an Electrolux 551. The belt moved toward the motor, cutting through plastic - and the soleplate - as it went. In my case, the Electrolux repair man replaced the motor with the type used on the 600 Series, with the bulge and end-washer.But it occurred again. The cleaner was quickly jettisoned.