The value of the gun will depend on several things. Condition of the gun as well as the ability to fire will drastically increase or decrease it's value. The only way to properly quote the price would be to take the firearm to a knowlegeable dealer or shop owner.
Wall hanger to 125 USD or so
there is one made by I thaca, model nitro special, sold under the name Lefever manufactured between 1922 and 1948, hammers are not showing. RG
Your gun was made by the Dumoulin firm in Belgium between 1885 and 1905. Value will be under $250.
Check the receiver, barrel(s), under the barrel(s), under butt place.
Under $100.
The current value of an ithica gauge over under with vented rib barrel and auto injectors is $1200.
50-100 USD or so
Do you know the model number? Are the barrels side-by-side or over/under? Does it have exposed hammers or internal hammers? What gauge is it chambered for? What condition is it in? A really rough guess at value for a plain, field-grade Ithica side-by-side 12 gauge made sometime after WWII through the 1960s would be about $200-$300. I have no clue what year your gun was made based on its serial number. Maybe somebody else can help with that.
20 gauge marlin ou 2 triggers
Best left to a gunsmith.
No way to answer without knowing barrel length and gauge.
It is found under the breach below the barrel