Don't worry about telling them you need a couple of more days to think about it, they'll wait. All of the advice given by the other agents is good as far as checking him out as a buyer.
But consider this, $13,000 down is a good amount. First of all, I would ask for a higher interest rate. Even a bank would want more than 5% on such a small amount for such a short term. 6 or 6.5 sounds better.
If he defaults, you get the property back! I know it would be a nuisance, and cost a little in attorney fees, but financially, your best case scenairo is if he pays a year or two ( monthly payment X months paid+ $13,000) and then he quits paying and you foreclose. You keep all that money, plus get the property back to sell again!
I just closed a deal for buyers who paid 6.5% for 25 years and were happy to do it. The seller had sold the same property in 2001, collecting a $25,000 downpayment, and 9 years of payments at $1,100 per month. The buyers quit paying and he foreclosed and sold to my clients for $140,000, with $28,000 down. He made $143,000 over the 9 years. He owed back taxes that they hadn't paid, but even with that and attorney fees for foreclosing, he cleared over $130,000. AND he got to sell the house again.
Just know that if you are willing to finance, you can ask for more in price and/or interest rate. Make sure that they keep insurance ( you should be on the policy as the mortgagee) and check to make sure they are paying taxes every year. If they aren't..foreclose!