There are a couple for sale on eBay right now.
The one: https://www.ebay.com.au/itm/1988-50-Cent-C-O-A-Bicentenary-Mule-Coin/184492742142?hash=item2af49fd5fe:g:-~MAAOSwEOhfi6GG
And one of the silver proofs as well: https://www.ebay.com.au/itm/1988-50-Cent-Bicentennial-Mule-Silver-Proof-Coin-PCGS-MS63-Only-Two-Registered/184546905415?hash=item2af7da4d47:g:Xj8AAOSwN5hfubxH
The seller seems to have a number of extreme rarities and in superb grades. How on earth does anyone (outside a mint employee) put together such a collection unless they have great contacts? Prices are too steep by a factor of 10 so I doubt they will sell.
Yes,extremely suss but you never know,the larger Dealers have someone always looking for Mint errors but it leaks out as soon as they try to sell.
I remember something about this on a Stamp Forum in the early 2000,i don't think i've ever seen one.
https://www.coincuriosity.com/view/1988-coat-of-arms-mule-50-cent.html
I thought a CAB issue last year said one person bought the 1988 50c mule and the 1992 Mob of Roos dollar but I don't see the 1992 Mob of Roos dollar for sale.