I am trying to sell it but it leaks gas from underneath and it is usually a lot. What exactly is wrong with it?

If it leaks when you start the vehicle, then the pressure line is rotten or the return line..

Could be an external fuel pump. or the lines going into the fuel system may be cut, cracked or dry rotted if its done alot of sitting around. Anyway l am surpised you havent flipped this thing by now. Those are like 70 times more likely to roll over than the avg passenger car!!!!

Most likely a bad fuel line, or gas tank inlet/overflow hose, both easy to fix, - also check the fuel lines at the carb/injection - - Oh and as for the comment about flipping it, Little known FACT Consumer reports had to print a retraction about the Samurai is seems they did not flip like they reported. (doctored account)