What symbol represents zero in the roman numerals?

They didn't really have one.  They had a word for nuffin, but it wasn't a mathematical symbol.

They knew it existed, they just didn't call it a zero.  They could easily count past nine, but instead of 10 they used an X.  Instead of 100 they called it C.    A thousand was called M.  If you had 200 Roman gold coins and the guy next to you had CC gold coins, you'd both have the exact same number.

They stopped using Roman numerals when they noticed their credit card statements were getting really hard to read, so they switched over to the Arabic numerals we all use today.

There is no symbol for zero.  Like the Greeks, "zero" was not a concept that the Roman's developed.  They also had no symbol of negative numbers as they did not believe that such numbers could exist.  

There was no numeric symbol for zero. When writing the numbers the place where a zero would go was left blank.   The Romans also used an abacus for counting.  Look up how to use an abacus and you will see how the Romans would have dealt with numbers like 304.

There wasn't one - they used the word nulla to mean nil.