I bought a Tamron 28-75mm f2.8 af lens for an all around lens. I use it for portraits, mx shots, and just general photography. Im pretty happy with it, and it didnt break the bank when I bought it new. Let me know how much you're selling your lens for and I'll spread the word around here as well.
You might try posting a add here http://www.thephotoforum.com/forum/buy-sell/
I'm not sure if they have a minimum post count clause before you can place a classified add but if they do I could make the post for you.
For anyone considering this lens I would not hesitate to purchase it. I use a Sigma with the same specs on my Sony SLR's. This range is perfect for MX shooting so you won't have to be changing lens at the track and risk getting dirt inside the body.