I don't know if it's happened here, but it happens to people all the time, people like my mum who use the same email/password combo on everything. If that information is leaked, then technically a third party can access lots of your information. Nowadays most sites require extra steps when doing anything sensitive such as making a payment (for example all our transactions happen via Stripe or Paypal, each requiring it's own layer of security) so generally it's more of an inconvenience nowadays.
However I have seen cases where accounts have been hacked and items have been offered for sale fraudulently, trying to trade on the reputation built up by the seller previously. Usually easy to spot, it's happened to my friend on Facebook. Again I don't think this has happened here, at least I have no record of it.
Another popular one is to list items that someone is genuinely selling as your own - happens LOADS on vintage car adverts on Facebook. Practices like putting a piece of paper with your username in shot (somewhere that it can't be easily cropped out) help against that.
We recently changed the Wanted forum to make it visible to paid members only, because that attracted scammers to message people saying they had the item in question (usually for a bargain price).
I feel that things have stepped up in the last couple of years, so we need to protect ourselves and pull together as a community to fight these knobs.