Your question is somewhat broad and non-specific, and as a result it will be difficult for anybody to give you much in the way of helpful advice. It is akin to someone asking what kind of car they should get, based on specifying only that it should run on "regular" gasoline. A bit more information about things such as what kind of parts you intend to mill, how big they are, and how many of them you need, might help to narrow down the field. However, you will find that it may be more productive to first take some hands-on classes in manual machining techniques before making any inquiries about what kind of CNC machines might be suitable for your work.
That said, $1200 would not ordinarily be considered anywhere near sufficient to purchase "a decent CNC mill to start a business". Perhaps what you have looked at on eBay is a CNC router, which is a different class of machine entirely. Whether or not such a machine would be suitable for you would depend on factors such as those described above, as well as on the quality and capabilities of any specific machine.
If you are truly considering getting a small "turnkey" CNC milling machine, one that might be usable for creating prototype parts as well as for doing small production runs, you might find the following engineering/design considerations document from Tormach to be a useful starting point in thinking about the capabilities to look for in such a machine:
Engineering Documents - PCNC 1100 | Tormach LLC | We provide personal small CNC machines, CNC tooling, and many more CNC items