Spanish 8 Reales Cobs - 1677 Potossi And 1733 Mexico

Both coins appear to me to be cast fakes.

First warning flag is the porous/mushy look to the surface, the coins don't have the sharply struck details of a normal coin. On the "royal" this is very obvious on both sides of the coin. On the Klippe (which is what the 1733 is, not a cob...different style of coin)the details are all too mushy. Most all Klippe type coins are very sharply struck from very detailed dies, the Spanish were transitioning between the very crude look of the cobs to the cutting edge technology of the milled columnario type coins (pillar dollars and the pillar style minors)
Second thing to check is the weight, although that can often be deceptive, my guess is these two coins are underweight beyond the normal tolerance. If you are interested in Spanish Colonial coins, you will need a scale, at least good to .1 gram, ideally to .01 gram. Thirdly, with a good powered magnifier, look for any evidence of either a seam or post strike grinding on the edge of the coin. If the coin is cast, there had to be a spot where the metal was introduced to the mold and it would leave a mark of some kind.
Lastly, read all the books you can, study all the coins you can, exchange information and questions with other collectors and by all means read every post Swamperbob makes on this forum. I can email you photos of a Klippe coin if you want (send me a PM) so you can see the difference in details but the photos are too large to post here.

Looks like I still have a lot of reading to do (actually I love learning, that's not an issue) :)
I'm starting this new collection (spanish and colonial coins) - after 15 years without touching my previous collection (back then it was different kinds - french coins from the eighties till euro)
I've ordered a scale, as I needed it to test some pillars anyway (and try the famous SG test which I've discovered here). I'll order a nice magnifier as well - as what I got is more for reading than for checking coins out.
I actually had doubts when I saw the coin side, which was pretty 'clean' compared to the coin itself (I suppose sea salvaged coin look the same on their side as on their faces)
Their price was attractive (that should have rang a bell) - but I thought it could be fine as the seller was not into coin selling and had other good looking expensive spanish (way more) coins.
It was easier to collect cheap coins, as there was no copy issues (mostly) :/
I've been learning a lot here, I'll continue checking out posts. This place is a gold mine, thanks for all :)
Oh, if anyone know a nice coin dealer in Paris, I have no clue where to start checking :/ (I've been to the gold trading street, but looks like they are more into bulions trading ...)

jfransch : I didn't manage to send you a mail from the forum (it says I'm not allowed) - here is a temporary mail : d93wbmxr29b0vt0 @ jetable.org
Thanks in advance !

Coin has been returned, and after about 3 monthes, I got my money back from ebay.
Lost a bit with the EUR/USD exchange rate, but at least I feel better than with my pile of worthless metal :)

Thanks all for your help in figuring that out :)
I'm now hunting for a nice 8 reales Klippe (the royal strike is too expensive)