I have a front projector and I am wanting to be able to use it to watch TV. I was wondering if it would be possible to purchase a cable box from eBay, etc. to act as a tuner. I currently have Comcast basic cable and am not interested in paying for digital cable. I AM NOT LOOKING TO STEAL CABLE. Just to be able to use my projector as a television to view the channels I am currently paying for. Are there any boxes in particular that will work for this? I need composite or S-video output. Colorstream would be great but I don't know if such a thing exists. Is it even possible or is my only option to subscribe to digital cable from Comcast and rent a box from them?

For purchasing a cable box from e-bay Short answer no!
For the analog cable stations you can use a VCR or DVD recorder.
For analog and unscrambled digital stations a QAM tuner MAY work for you!

You can purchase a cable-ready tuner (analog) from places like Radio Shack, which will serve your purpose. However, it will probably be cheaper to just use a VCR. But be warned - some VCRs on the market today don't have any tuners at all, so check the box before purchasing.

I suggest getting the cable box. It is not much more then expanded basic. You have two advantages using a box as your projectors T.V. tuner that I see. First, you will get a better picture. You gain a digital stream and have the use of better cable types, (interface), like s-video, etc. Second, the ease of remote controlling channels.