The quick answer is that yarn for machine knitting comes in many weights, the same as yarn for hand knitting.
But much of the machine yarn sold in cones on eBay was originally made for commercial knitting in factories. It tends to be very thin. You can use it for things like lightweight lace shawls; you can use several strands held together; or you can combine it with other yarns to make one thicker strand.
The eBay listing should tell you the gauge to which the yarn knits and the fiber content (eg, wool, cotton, viscose, etc.). If it doesn't, you really don't know what you're getting.
Here is my unbreakable rule for buying yarn on eBay:
the listing must contain gauge and fiber content, which indicates that the seller knows something his/her merchandise. If it doesn't, the yarn is probably being sold by someone who doesn't know anything about yarn or knitting -- not a reliable resource.