Museum Park is an area in the town with some relics of Marree's bygone days. It gives an interesting insight into the past and how the town evolved. there is also a camel built out of railway sleepers which acts as a sundial.
Visit the Arabunna Centre. They have a collection of local Aboriginal artefacts and a lot of information. The people are very friendly and helpful. There are also souvenirs.
What makes it so special? If you don't get it here then you'll have to go many 100's of kms before you get the chance again.
What to buy: Food and fuel; post office facilities.
What to pay: Prices fairly high as to be expected in remote country.
The essential ingredient for a great and successful outback trip is BEING PREPARED. That includes you as well as the car. I prepared a piece on safety in the outback which is worth a read if you intend to travel in this area, although it also could apply to many other areas. It's on my Australia page but click on this Outback Safety link for a shortcut there.