Roland Perry wrote:

I believe it's also illegal to sell/transfer the cards, so someone else
who happens to be visiting NY can use up the money.

It most certainly is permitted to give or lend a MetroCard to a friend
while you're not using it - even an unlimited card. (Of course, that
only applies to standard-issue cards available to anyone - Student
MetroCards and Reduced Fare MetroCards and employee passes and the like
are not transferable.)
--
David of Broadway
New York, NY, USA