This is outside of my usual wheelhouse, but I found the listing interesting. I do have some doubts about it's genuineness/provenance.... but that's perhaps for another discussion.
This was consigned to Iconic Auctions but it's also been on eBay for awhile listed at $17995.00 OBO.
It was featured in the recent Iconic sale that ended yesterday and had a starting bid of $500.
I thought to myself, "...no way is this going to fetch close to the hoped for eBay price". That hunch was right.
The 'final price' on Iconic was $4579.29 w/BP. I'm thinking that the term 'final price' is not synonymous with the traditional understanding of a 'winning bid' when it comes to Iconic results.
It is odd because the "proof photo" shows the dummy, not Linda Blair, holding a cross. I am not convinced it was Linda Blares hands in the other shots. I recall they had doubles for some "distasteful" scenes - as in certain scenes Taxi with Jodie Foster's sister and DeNiro. The scenes I remember with the live Blair holding a cross in this sequence were fast and fleeting with much movement. How many were there is another question. These sources are rife with games as another recent post showed.
A Jesuit friend of mine had a crucifix in that style. I believe they were worn regularly in days gone by. You can see the loop at the top for a chain so it could be worn. There is nothing unique about it as an alleged artifact from the film.