If/when they go past the legal deadline send them a letter notifying them you intend to file a complaint with their licensing board, or what have you.

In this situation if I would be him I would call for highest and best by a deadline and not put the property under a contract with wholesaler.

He promised  for the third time that he'll  he'll meet me  on Wednesday, the 10th day after signing contract and will send money via money gram but I refused because It's specified on the contract that security deposit, first month's rent should be in a form of cashiers check and electronic transfer won't be accepted since the company that will process payment don't accept electronic transfer and that he did not honor the deadline I gave him.  

I could also put a bed and couch in it before my deadline ?  

Low risk is still a risk.I hope you meet your deadline, but don't revolve your first deal around it.

I explained to him that I have interest in other properties and gave him  a Thursday deadline  (been going on 3 weeks )We as investors can't wait forever and have limited resources. 

My question is: Since the deadline has come and gone to sign the extension lease, could I now serve them a 60 Day Notice to Vacate?

They never met one single deadline and ultimately payed our car off late even though they swore that it would be sent several days prior to close, it was never even sent - luckily it was the only late in the etire history of paying off the car so they backed out the late fee and fixed the records to reflect on time payment - working wit Tri-Counties was an absolute nightmare.  

Well, you can . . . but when the seller finds out the cash is NOT your cash, they could pull out of the deal, and depending on your deadlines,  they could keep you earnest money.  

Send the required itemized deposit adding in rent/charges etc with a deadline to pay stipulating they will be sent to collections if not, send them.