8 Reales Cob Coin? Help Needed Please!!

Yes, Mexico mint, assayer D (1618-34)... very typical strike and shape for the period. From its "look" due to the style of "conservation" (cleaning), with the distinct light pewter/greyish hue, it is likely from the Lucayan Beach (Bahamas) wreck of ca. 1628. Seeing your location in the UK backs this up, as most of these surface on your end of the pond - they were marketed by Spink's in the mid-late 60's.

Moderate corrosion (and accordingly maybe a tad more polished than the Spink's pieces are normally found), but well-struck/intact central detail... In general, the equilibrium sell-price would fall in the USD 100-150 range, I'd say. Now, people can have vivid imaginations when it comes to Reales coinage since they detect the "crossover" appeal beyond plain old coin collectors - colonial significance, trading history... (deep breath)... pirates. Thus, some ebay sellers will shoot for the moon with their price hoping to find the sucker who occasionally will surface. So, if you're inclined, you could always start out at something higher than that stated range and see what kind of response you get.