You might want to keep one of your machines and trade in the other for the new combo machine. Embroidery projects do tie up the combo machines so I also recommend keeping, even just a basic machine, in addition to a combo embroidery machine.
I didn't want to deal with public u sell sites like craigslist or even yahoo groups... too many risks now so I traded in two of my machines toward a combo machine with a dealer. Really glad I didn't trade in all of my machines though... comes in really handy when my combo is tied up doing embroidery projects. Saves a lot of time and trouble.
A local quilt guild might be good way to sell your machines if you don't want to trade your machines in...you can sell them at a guild (if they will allow) and let a potential buyer see that the machine(s) works, with witnesses to that fact; not to mention a safer place to be.