Question about theft

[FFrankie]

What am I supposed to do if I find someone selling a coin that was lost in the mail and never returned to me but now some random person is selling it.

[sam00]

What am I supposed to do if I find someone selling a coin that was lost in the mail and never returned to me but now some random person is selling it.

It really depends on where you live. In my country for example you would still be the rightful owner of said coin and anyone selling this would not be entitled to do so as it is written in the law.
Did you buy or sell the coin that was lost ?

[bL4nkcode]

Returned, you mean that the delivery was not successful and the package should be returned to you? You as the seller?

Then, you should complain to the post office or the responsible logistics, AFAIK it should be under the seller protection and you should be compensated as much as the declared worth of the item.