If you are trying to establish an appropriate price, we can't help in any meaningful way without pics. Value will be based on original quality and current condition along with the brand value of Mahillion.
Mahillion made some very nice guns, FYI.  But the NA market doesn't assign a lot of value to the higher quality Belgian makers, relative to English or American makers names.
Here's a list of the top makes from Liege, Belgium. Guns with these names are definitely not JABC guns...... Bernard, Brancquaert  Britte, Jules Bury, Christoph, Courally, Defourny, Donckier, Francotte, Forgerone, Galand, Janssen, Lajot, Mahillon, Masquelier & Thirifays.
On average, a gun of equivalent original quality and current condition by one of these makers/names will be worth half of what the same gun by the top London makers (Purdey, H & H, Boss and Woodward) will fetch. Branding is important! 
edit to add: BTW, that "half the value of a London gun" isn't some guess on my part. It's the result of a long term (10 years plus) analysis of actual auction results in North America and in Britain. 
I'd be interested in it myself because I love Belgian guns but.....two strikes....it's an O/U and its a 20 gauge. Don't buy either.