Lynn, as stated above, there are many kinds of CNC machines. You did not state whether the machine you are looking at is a lathe or a mill. There are CNC machines designed for shops making parts, and some are designed for jewelry, where the piece is very small and a high degreee of detail and precision are required. Roland makes CNC mills for jewelry use. Most CNC jewelry is cut from machinable WAX and the wax used to cast the actual part.
To do engraving you need a high speed spindle. That is what holds the cutting tool, which is like a miniature router bit. Such spindles turn at very high speeds, 25,000 to 50,000 RPM is very common. The spindles that come with most mills run at about 6,000 RPM max. The runout (tolerance) in these spindles is often less than .001".
Of the two types of machines, in my opinion, the mill with 4th axis is the most useful since it can make engraved round parts as well as flat parts like clips.
Using as an example an engraved center band, the first step will be to draw the band using a 3-D CAD program. Then the CAD file is run through a CAM program to set the tool paths. The CAM file is then used to run the CNC machine and do the actual cutting. The prices of CAD/CAM software range from free to thousands of dollars. The problem is that the 3-D software suitable fo jewelry (pen parts) is not the cheapest!
Given the above, even if buying a used machine that is suitable for pen parts, the total expense before you are ready to make the first part will probably be a few thouosand dollars. You will need to learn CAD/CAM to design the parts and produce the CAM files.
Hope this helps. And I hope you can get the machine if it will do the job.
A CNC lathe will cut anything that is round. To engrave around a cylinder, such as a center band, you need a mill with a 4th axis. The 4th axis is a CNC controlled rotary table.