It’s not uncommon for the bid to be zero percent, the tax lien buyer making his money on obtaining the property and selling it if there’s no redemption. 

It's uncommon to pay STR cleaners by the hour.

@Conrad Meier, this is not uncommon with a tax sale property. 

It is not uncommon that a mortgage contain a provision providing that in order to transfer any interest in the property, (including to a real estate holding LLC) the bank must consent.

Many companies will do notes or bonds that are unsecured debt against company as they don’t want to pin it to every asset which is not uncommon - but when they take on additional debt that is secured and/or start to incur massive losses that’s when it gets hairy.

It sounds like it isn't that uncommon though.  

That's a bummer you had this experience with a seller although it is not uncommon.

While it's not uncommon for buyers to back out of contracts, especially in real estate, it can be particularly challenging when it happens unexpectedly.

So, just like any other sale it allows them to require the mortgage be paid.However, it is very uncommon for them to do that as long as you keep paying the mortgage.

These situations are unfortunately not uncommon.