I have a POD 2.0 currently, I've had it for about 6 months. It sounds....OK, I'm really into heavy stuff with a lot of distortion. The insane setting is OK, but I'd really like to have a more professional sounding tone. This may be due to the fact that I'm recording from a 1/4" to USB cable into Audacity...then again, I just don't think the 2.0 does it for me. The only problem about upgrading to the POD X3 is that I don't have the money (I'm 14, my birthday's passed, and Christmas is still far away). Would it be worth it to sell my POD 2.0 to Guitar Center for some extra cash? It's in great condition, I still have everything that goes with it.
No. Do not sell to GC or any place.

Sell on CraigsList locally or even do Ebay if you could (considering your age, craigslist might be iffy with your parents considering meetings with generally older people).