There are some questions posed here that really should be answered by your Realtor or perhaps their broker.
I would also recommend discussing this with your CPA or tax professional.
I am puzzled why the buyer wants to purchase the property by installment sale rather than a mortgage. That would be question number one.
Second question is that if your home did not sell in six months, don't you think that some homes have sold in the last six months? But yours did not?
In most cases homes that do not sell are over-priced. Remember , it is not the amount you owe, nor the amount you need to net that determine the price. If this is a case where you need to sell, but if you did sold at market price you would come up short, then you have a different question.
I would suggest talking with your REaltor, or their broker, about your options. The only reason I can think of for an installment sale is that is the only financing they can find. If that is the case, are you sure you want to sell to this buyer? In most cases lease to own is problematic, most buyers are not interested in locking in a future price in a declining market. The advantage of a lease to own is that you are getting an premium for allow the buyer an option to purchase the property. Depending upon your market conditions you might be better off just renting it out.
If you are upside down financially, then renting it out may be your best bet. Having a home on the market, vacant, for six months does not sound like a good plan to me. I went through the same thing. At first having a rental property was streesful. In the long term it turned out to be a great decision.
PS Your profile shows you in Atlanta (see link below). Your picture says "lancaster". FYI