How Do I Set Up A FB Business Account / Sell On Marketplace

[KTMsm]

I know it seems simple but I've googled for ages

I've got a personal account and I've set up a "business page" but I can't see how to post as my business rather than personally nor how to sell items on marketplace (which I can do personally)

When I google I just go around in circles as it thinks I want to advertise a business service via FB - ie a pop up ad, but I want to advertise motorbikes in the classified ads (as many other traders do)

I'm sure I'm missing something obvious