Wraparound Mortgages

I have a rental property that just became vacant. It currently has a non-owner occupied 1st mortgage on it. I had someone contact me that is interested in purchasing the property with seller financing. He said he typically does a 5 to 7 year deal, with the intent on selling the property during that time, then paying the balance due. I expressed concern about if my mortgage would allow me to do this. He said that it should. Before getting too deep into the negotiating process, what should I look out for? I know my basic risk would be if he defaults, I would personally need to foreclose on the property. How much would that cost me?