Realty Transfer Tax. 

If they charge more its considered an illegal tax.

Airbnb issued me a 1099-K tax form, indicating I meet the threshold for gross rental proceeds paid by guests.

Does time taking care of (mowing/landscaping) and managing (keeping records, paying HOA dues, paying taxes) my vacant parcels count towards my 750 hours of being a real estate professional?

If there is something special in your scenario that is causing the issue you always have non-QM products to fall back on like DSCR (qualify based on the asset, only works for investment properties) or Bank Statement loans (qualify based on income, no taxes required, great for self employed).

Do remember when thinking to MTR to calculate in 7 days and less avg stays to qualify for material participation when self-managing STR for your tax year deductions as a business against your W2 wages.

I know there are certain liability protections, but I'm more interested in the tax implications.

My Property Manager mentioned sending this to a collections agency but said there was no gaurentee that we would ever get the money owed.  

Better to start on sound legal and tax footing.

You can Sell you Home Tax-free for up to $250,000 if you live in it 2 out of 5 years.