I assume from your question that you sold it to the buyers, and retained a lien against the property, but leaving your original mortgage lien in place. If that's correct, and the buyers are not paying you, you can foreclose your lien. In the meantime, you will also have to make sure that the payments on the original mortgage are being made, or they will foreclose, leaving you with nothing.
In order to foreclose, you will need to send proper notice, check for tax and/or mechanic's liens, post the property for foreclosure, conduct a foreclosure sale, and prepare a trustee's deed to the buyer at the sale. The buyer at the foreclosure sale will take the property subject to the original mortage debt, and possibly other liens as well.
I recommend that you have any foreclosure performed by an attorney, and I can refer you to someone if you are interested.