Gold was money, and fair market exchange was the logic used, currency for currency, $35/oz
Gold bullion was/is a commodity. Minted coins made from gold were a type of commodity money, as opposed to representative and fiat money. The former was property, the latter legal tender. Both were seized.
And no, they were not paid fair market value. In 1933 the value of gold was $26.33/toz., however people were paid $20.67/toz. for their gold.
To put it in modern terms today gold closed at 1,697.00/toz., now imagine that tomorrow you were compelled under threat of criminal prosecution to drive down to your local municipal building and turn over your private holdings of non-coin gold, and they decided to only pay you $1,332.21/toz. for it?
Exotic animals that have been banned, but were in legal possession before the ban were not destroyed or confiscated.
Still hunting for the news articles, but I recall several localities banning species such as pitbulls and compelling the owners to remove them out of the locality.
And to address DS's question, my point remains, the reason I ask is to put in context that a confiscatory law would be treading on really new ground.
As distasteful as this is, I feel need to point out that at one time it was legal to own a human. They were the legal equivalent of property. Then a law was passed to make them not property.
Regardless of the situation, the former owner at time point A was the owner of property considered to have certain value, then at time point B it was declared that he no longer had ownership of that property and his personal wealth was reduced by that value (he no longer owned the slaves that he had invested a particular amount of wealth into and was not remunerated for that lost wealth).
That the property in question was a sentient creature and never should have been property to begin with does not alter the fact that the process involved declaring that a particular type of property (slaves) was illegal after a particular date in time.