Make sure you can lock-in the rate on your securities-backed line of credit / margin loan at a rate where you are at least break-even with cash flow.  

Check with your mortgage broker to see what you may need to provide if you are on the deed... but I'm reasonably sure you will be asked to sign the security instrument (mortgage or deed of trust) since this appears to be a requirement in the Fannie Mae Selling Guide.

Conventional guidelines requires a lease, and no security deposit.FHA requires a lease, security deposit and proof of it being deposited. 

based on the information you provided, I would give it another shot and secure another tenant, and get a HELOC/cash out refinance to get another property.

Sometimes, a home warranty offers valuable coverage that you can use with no hassle; other times, it just provides a "sense of security" when a first time home buyer finally buys a house.  

This almost naturally ends up being a lower rate of return for security. 

There is nothing different about a contract with seller financing other than the section regarding purchase price and financing.Here's a basic provision:Seller agrees to hold a purchase money deed of trust and note secured by the property in the amount of $x at a fixed annual interest rate of y% payable in equal monthly installments of $z... with the full unpaid balance due, if not sooner paid, on [date of balloon payment, if applicable]

Since VA loans are residential mortgage backed securities, a commercial loan can't be utilized.

This potentially caused us to lose out on a few other tenants who were ready to begin the lease ASAP, but we decided to go with this couple because they had better credit scores and financial situation.Does anyone have any advice on ways to prevent this from happening, such as including a fine for people backing out or requesting the security deposit upon signing of the lease?

Do you have a security deposit?