Your description of a no serial numbered side-by-side double shotgun fits the product of dozens of small shops in the Eibar region of Spain. For centuries, this was the arms and armor producing center of the nation. Your gun w/o serial number was imported before the 1968 Gun Control Act that required serial numbers to be stamped on all firearms manufactured or sold in America.
Value is probably in the $100 area more or less. $70 sounds like a real bargain unless the gun is defective. You might want to give a very carefull inspection for barrel, firing pin, action, or stock damage.
Removing the black spray paint is relatively painless. Use mineral spirits (paint thinner) and fine 0000 steel wool. The bluing underneath will not be harmed. For faster removal, a more aggressive solvent such as lacquer thinner, acetone, or even paint remover will work, but the stock finish will be damaged.
HTH
John