Palladium is worth more than Gold and it might be confusing to name the lowest valued level with the highest valued metal.
It's hard to explain in text how the exchange works. It would be closer to a marketplace than an exchange in the same way that Steem monsters are. Say you what to buy a "red crypto card", you would go to the market and select on the red crypto card. When you do that a new window would open and all the cards for sale would be shown in a list.
The use case of an investable collectable card would is to HODL it, the game is HODL and make money. I don't understand what you mean about liquidity? Collectable tokens could be bought and sold to other people on the marketplace so SPinvest would not require any liquidity.