When a shortage of a good becomes evident especialy due to over regulation and ways are available to to get the goods and get around the regulations under which they are sold.
Of course you can the whole point of the black market is that it's illegal and when you do get arrested you will be sentenced to a long time so best to get out while you can.
You can't sell your pony if it is on the Grasslands. You can't sell your pony if you have 3 or fewer ponies. You can't sell your pony if your account is less than 10 days old. If you meet any of...