If you did not have them sign a note and a security instrument that will make it 10x tougher.

It's an amazing instrument IMHO.I should mention - an experienced investor friend told me he only gets mortgages/refinancing on properties, never HELOCs, because any line of credit limit can be shrunk down suddenly by the lender, any time they want to. 

They must be notarized, the notary attests to the fact that the persons named in the foregoing instrument were in fact the persons exeuting the document.

A judge would have to decide if the payment method was an "other traceable or negotiable instrument." 

According to Ron LeGrand in his state of Florida, the instrument that is used is called a wraparound mortgage. 

It can simply say: ASSIGNMENT, For and In Consideration of four percent of the Option Price hereinabove mentioned, the receipt of which is hereby acknowledged being paid in hand by the signatures below, the Optionee and Holder of this Option Agreement hereby grants, bargins, sells and sets over unto _____soandso, all of Optionee's rights, title and interests, at law or in equity, in and to this Option Agreemeent as of the day and year written below.ENDORSEMENT: ___________(and the Optionee signs) Date___You may have the buyer then sign as an acknowledgment.I would not file for record the actual Option Agreement, but a Notice of Option that would not contain the option amount to buy or any other considerations paid, but it would include the term of the option.Now, I'm sure that many investors have all sorts of assignment contracts for the sale of the option and disclosures but a simple endorsement with the assignment stated is all that is necessary to assign an instrument, much like endorsing a check over to another payee.

(i) the term "Insured" also includes(A) successors to the Title of the Insured by operation of law as distinguished from purchase, including heirs, devisees, survivors, personal representatives, or next of kin; (B) successors to an Insured by dissolution, merger, consolidation, distribution, or reorganization; (C) successors to an Insured by its conversion to another kind of Entity; (D) a grantee of an Insured under a deed delivered without payment of actual valuable consideration conveying the Title(1) if the stock, shares, memberships, or other equity interests of the grantee are wholly-owned by the named Insured, (2) if the grantee wholly owns the named Insured, (3) if the grantee is wholly-owned by an affiliated Entity of the named Insured, provided the affiliated Entity and the named Insured are both wholly-owned by the same person or Entity, or(4) if the grantee is a trustee or beneficiary of a trust created by a written instrument established by the Insured named in Schedule A for estate planning purposes.

However outside those instruments you can do the following: Run the comparitive numbers and see which works better for youAverage Turnkey Cash Flow Per Door In Phoenix Metro Area No Bank Neededhttps://www.biggerpockets.com/forums/600/topics/584916-average-cash-flow-per-door-in-phoenix-metro-area

It is one of the oldest instruments around and institutional lenders will recognize it as a form of ownership and allow you to refinance out of it.To protect from all the very valid concerns that @Steve Vaughan expressed, definitely run it through an escrow service.The primary risk is that you could be evicted like a tenant instead of foreclosed on like a borrower.  

Spending time on this site was instrumental in helping me to navigate that deal.