Most of the "IIfx" machines that you see for sale on ebay are actually upgraded II/IIx machines. You can see in the photos accompanying the listings that the cases have Macintosh II or IIx on the case fronts. A "real" IIfx is harder to find. Upgrading an older machine to a IIfx by swapping motherboards was cheaper than buying an actual IIfx so many users of the older machines opted to go that route instead. It was like upgrading an Apple IIe to a IIGS, but in that case the upgraded IIe's are actually more than rare than actual IIGS's because not many people saw the advantage and the Macintosh was already on the horizon at that time. 

I bought a IIfx not long ago even though I figured the board would be destroyed by cap and battery rot just because it was in an actual IIfx case.