Welcome to the forum Mpoulin!!! Beautiful knife and sheath!!!
here is what I know based on my books and research on my example I have...
The knife you posted is a CASE TESTED 551. As others have stated, the knife is rare and the sheath almost rarer to have with the knife and not having been sold seperately or lost. Your knife has the "sans serifs" stamp. I believe used around 1930'.
My knife is the "serifs" stamp, also circa 1930'. Difference of course is the font used on the "CASE" stamp of the tang.
I can't see your knife pic very well but from what I can tell, the blade is looks full and if sharpened, it is minimal. All this affects the value. Some side play to the blade is normal on these knives, when blade is opened and locked, side to side play measured at 1/16 to 1/8" maximum at the blade tip is normal.
Some blade scratches are expected from the top and bottom liner locks as they work independently. The lower liner lock (away from the blade pivot) releases the blade form the closed locked position and the other liner lock, locks the blade open. Close the knife almost all the way and you can see what I mean. My knife has some too.
Your knife also has some really beautiful stag. Here is MY OPINION on value, others may vary...
The 551 with the "sans serifs" stamp like your knife lists for $1200 in the price guides.
The 551 with the "serifs" stamp like my knife lists for $1400 in the price guides.
Keep in mind the price guide value is for a factory mint knife and not everyone uses the guide for value. Ebay is a whole different ball game I know nothing about as I don't do ebay.
I bought the "serifs" knife and sheath in the pic below for 800 dollars from a dealer. Another member on this forum has one with the sheath he says he picked up for 500 bucks.
Again, just my opinion. It is a real pretty knife and rare but my guess is that if you get 800 bucks out of it, you are ahead of the game. You may get more, I hope you do. Just depends how fast and how bad you want to sell it and how bad someone wants it. On ebay auction, maybe 400 to 600 bucks depending how crazy people get as these don't come up for sale often. Don't know for sure, too erratic and too many variables for me to gauge.
As others have said, Great GrandDad's knife is priceless, so if you don't have to sell it don't. But if you have a hole in the roof that needs fixing, do what you have to. If we are talking home improvements and not needed repairs, then I would take my time and get the most bucks possible if you choose to sell.
Hope this helps. Here is the pic I mentioned and a link to some additional info and pics....
http://www.allaboutpocketknives.com/kni ... hilit=+551
Regards, Jerry