I need a new charger for an old macbook pro. If buying used or from another source, what do I need to know?

I am planning on replacing my MacBook pro 15" from 2013. I want to do it before it bites the dust. Even though it still chugging along, the charger is frayed at the connection. I see replacements online from places like eBay and Amazon. I'm considering one that is compatible, not made by Apple. What do I need to know?