Basically it just means any belongings, any stuff you might have. E.g my camera, laptop etc is some of my ‘stuff’, as are the half empty tins of paint in the garage. This is more of a personal thing, as in the scene, some people don’t like the idea of selling stuff, like it means you are broke and have no other options. I used to sell stuff I didn’t use any more just to get rid of it, or to upgrade, and my wife didn’t like it until I explained I wasn’t going to sell everything in the house ;-). However, in the scene above there is a play on words also - they are talking about selling ‘stuff’ and Dorothy’s mother either unintentionally or as a rather risqué joke hints that she thinks Dorothy means her body, her ‘stuff’ as in her female charms.

"Stuff" can include many, many things. About everything that belongs to you. For a young girl, that can mean her watch, a golden ring, maybe a very expensive bag, etc etc. For a young boy, his bicycle, his watch, etc etc