The Colt 1877 in .41 Colt caliber is known as the Thunderer but this name was from one of the major Colt Dealers in the late 1800s. As "JohnD56" stated " Dry firing is probably not a good idea for something that old". If the revolver is working in SA and DA, that is good. The major problem with the 1877 is the SEAR! This part is only working when cocked SA, and the Sear is made from Malleable Iron, heat treated, but the tips wear quickly and you lose the ability for SA firing. Use the DA as much as possible because finding an original Sear is difficult and they cost about $150 when you can find one. There are reproductions made, but difficult to fit correctly. The other problem are the Springs can break and must be replaced. The reproductions springs can be fit pretty easily. I would recommend that ONLY black powder cartrides be fired in the 1877.
As to the value, this is tough. Being manufactured in 1900, the wear on the stocks show the revolver has been used of handled a lot. So , I will be assuming some facts. The good part is is works in DA & SA. From the pictures, the finish shows wear. Assuming the bore is good with little pitting, it is all matching, not pitted on the metal and has a weak Ejector Spring (another hard part to find). The Ejector Spring from Numrich Arms will not fit, and I don;t know where to get another one. I would put the value at $800 to maybe $900. These revolvers vary a lot in pricing; even on Gunbroker. I like the Colt 1877 and can do some work on them, but since parts are difficult to find, I only buy those in working condition now. Here are a couple if mine.