We finally have a court order saying he has to be out on Monday.

So not sure worth prospecting the ones that get 1 case every year or two or three.....as some of those won't be sellers....maybe husband/wife survivors that stay in the house, kids move in, they already have a realtor, or some other excuse.You might get the 2020 list and run that for your county or local probate court and see if you can put that in Excel and shift and sort by attorney to see the ones who really are active and handling cases.

However, the problem is that the new property will most likely cost 2x the proceeds they'll be getting from the old property and since they are retired they most likely will not qualify for a mortgage to make up the difference. 

Assuming that you have a properly formed contract, the short answer is that they can refuse to perform, but there will be consequences.Those consequences normally play out in court, so it's not normally worth the cost to pursue it. 

What do you plan to do with the sale proceeds?

Your proceeds are... 70% of $100K = $70K minus closing costs (let's call them $5K) = $65K cash in your bank account. 

If it were mine, Loud boy one warning--then non-renew--or boot out.Car issue, number the spots and assign them in the leases with a parking sticker with the spot number on it.Staple a duplicate numbered parking sticker to the lease in case you ever need it in court--vs peeling the one of the renters car.Possibly do an addendum for this to the lease if need be.Post warning signs according to local law and set up with a towing service.Car issue with no sticker parking there, have resident have service tow it--according to local law.No need to get involved in the car issue then. 

They have every right to be there and if you go to court you will get your behind handed to you. 

@James Devoe  Yes one should always proceed with caution when the seller is in a hurry. 

At what point do I notify him of the late fee and or consider eviction proceedings?