I am a comcast subscriber and don't want to rent a DVR from them, i would rather buy one from e-bay. Does any one know if I do this first would it be compatible and second would comcast give me any trouble???
I assume you are talking about the analog channels on Comcast. The regular channels are not encrypted (yet, anyway) so yes any DVR should work fine on them. The scrambled ones would require a Comcast DVR since it has the decoder enabled.
As for the clearQAM digital channels, Comcast in my area (Washington, DC) tends to move those around a lot, so I wind up going thru the channel setup with them about every other month, 30+ minutes each time.
Your cheapest and most straight-forward solution (assuming you want to avoid using the Comcast DVR), is to get a Tivo, and then call Comcast and ask them to install a cablecard into it so that you can get the encrypted channels (you don't need cablecard to see the unencrypted channels). I think the cablecard rental runs somewhere between $5 and $14/month.
There are other solutions but they involve more work, and are not guaranteed to succeed.
I believe Tivo is attending the Jan 7-10 CES exposition so you may want to hold off until then to see if they introduce anything new.
I had the same thought buying a comcast dvr from ebay but my local comcast provider told me comcast doesn't sell their dvr's so if someone is selling them on ebay, they are units that never were returned to comcast. In addition I was told those dvr's would not be activated. I'm not sure how much of this is true or bs but there are dozens of units on sale at ebay.