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?

That being said if you already have a property picked out to roll the proceeds into there is an option called a reverse 1031 exchange.

From paperwork, to what contingencies (if any), and how to proceed in general once you have found a seller and an agreed upon price.