Is it legal? Technically, getting a carded mintmarked coin, taking the coin out of the card, replacing the mintmarked coin with a generic coin, then selling the card without mentioning the switch, would be fraud. Very very minor fraud, but still fraud. Especially given that anybody doing this is likely to be also on-selling the de-carded coin - in effect, they are aiming to double their money for zero effort.
Given that I doubt the cops would be interested in pursuing a fraud charge over a few dollars (unless the fraudster is doing this on a large scale, with hundreds of listings), perhaps the more pertinent question would be, "Is it against eBay policy"? Again, technically, yes, it is against eBay policy to sell fraudulent listings; under their Item Description policy, "Item descriptions must be accurate" and failing to disclose a bait-and-switch would be inaccurate. Getting eBay to understand the fraud that's taking place would be the hardest part, as they have no numismatic experts on staff. By the time you managed to convince them, the sale would likely be over.
