Hi Murray, caveat emptor loosely translates to "buyer beware" so I'm not sure that's exactly what you mean.
True, there probably are situations where people pay more on ebay due to some auction frenzy than they might in a more sane/rational situation, so, you're right. However, for that one particular sale on ebay the final price is the right price. Really, what I think you're talking about is, what is the general price for a particular item as a composite of all the various price points (ebay, retail, etc.). Hard to tell exactly, however, a downdraft inlet manifold for webers is a semi-rare and sought-after item likely to command a good price in just about any selling forum - probably several hundred dollars at a minimum.