IMHO, $350 for a new barrel is an amazing deal. Around here, I have never seen them below 450+ and (depending on the time of year) seen them as high as $700. At $350, I would be purchasing new every time!
IMHO, you take a risk with a used barrel. You do not know what was in it or how the barrel was cared for (when you buy used). To me, a used barrel is good for breaking apart and making chairs or side tables with the wood stock. That being said, I feel that no used barrel is worth more than $100.