It looks to be a extremely intruding and fascinating coin. I think it's a barbarous counterfeit (this isn't the first time i have seen an unofficial SPES PVBLIC) but looking at the edges it could also be a modern forgery.
I would definitely not want to spend much money on it until I had closely inspected it. There seems to be some convincing deposits on it, but these are easy enough to fake. The edges do seem a little off (they look almost sharp), but it may just be the lighting and the picture is also a little out of focus. A clear high resolution picture would help.
Below is an example from Ancient Coins Canada.
I bought the coin. Here a couple of better pictures.
I hope it turns out to be authentic.
It's hard to say from these photos.
By the way, the seller looks honest and i can return the coin to him. I want to think positive.
Yes, as long as he accepts returns, you have nothing to lose.
http://www.academia.edu/1469458/Highly_Deceptive_Forgeries_of_Constantines_SPES_PVBLIC_Coinage
In this case the host coin could be a VIRTVS EXERCIT.
I obviusly blocked the payment.