If you knew a colleague was thieving, would you just keep it to yourself?

Our office receptionist does stationery orders &i have been noticing since last yr, she orders some things we normally use &sell them on ebay. I have seen her take cartridges worth �300 wrapping them up &taking them to the post office, one day i saw one under her desk wrapped ready to go &it had her ebay username on it. At 1st i thought it was none of my business but she's doing it more &more, she clearly hasn't a clue i know. Just keep quiet &concentrate on my job, i'm just a PA?