Private money loan collateral

[Brook Rieman]

I am looking to get into buying flip properties at sheriff’s sales but I need cash to do that. I have found several friends potentially interested in providing private money loans but I’m wondering what you use as loan collateral in this kind of case. If I were purchasing a property directly, they could have a lien on the property being purchased but what about in a case like this where you have to have the private money in hand BEFORE buying the property? 

Edit to add- would it be wise to have them put a lien on my other rental properties which I already own?