I doubt you're going to find a deal on a used one that's compelling enough to beat the $299 that you can get a new one for now.
Who knows until the official pricing and reviews of the new one come out -- but I'd jump all over the existing lens at $299 regardless. There's not a lot wrong with the lens that would be worth an extra $200 or more dollars.
On a side note I just got a new Sigma lens (the 10-20) and the zoom and focus rings turn in the opposite direction of the 17-70, that's going to be so weird. I understand different brands having different zoom ring directions but you would think that Sigma would at least standardize on one direction.
That's what I love about Sigma you have to go the other direction....lol. Where is this lens for sale at for $299.00? I found it at B&H for $399.00 http://www.bhphotovideo.com …70mm_F2_8_4_DC_Macro.html
If that is not the right one please point it out to me. Thanks