Converting logs to lumber is one thing... converting lumber to cash is another. You'll never be able to compete with the big mills, so finding specialized markets will be the key. Trailer decking, barn siding, and fencing may be good options to get started. Added service, such as installing the trailer decking, air drying the lumber, etc. will also help out the bottom line, if you price it high enough to make a profit.
I bucked the trend and sold my hydraulic mill (Timberharvester) for a Norwood
manual sawmill (HD36), and it was a great move. Portable sawmilling accounts for about 60% of my business, and the manual mill is simpler to run, easier to get to the job site, and much more reliable than the hydraulic. Since there are no other portable sawmills in the area, I keep busy. And there are no issues about selling logs.
If you do go the used route, avoid any mill that has been left out in the weather. Otherwise you will be replacing hydraulic lines, fighting bad electrical connections, and dealing with a host of other problems.