Congrats!
Is it in shootable condition? If you're not sure, take it to a gunsmith and have the barrel slugged/inspected and the head space checked. These rifles are a hoot to shoot!
I have been getting my shells from Powder River in the low pressure loads (Don't shoot the more modern .45-70 Gov't ammo) but now that I am reloading I plan to buy the .45-70 die sets and reload my own.
Springfield Trapdoors were very popular in the late 80's and 90's, their values went way up, but now they've dropped again. Base on your photos, I would make an educated guess and say that yours is somewhere in the $400-$700 range, depending on how motivated the buyer is. These can be very hard to sell anymore on gunbroker.com.
Photos of my trapdoor, with 1879 improvements, cartouche is 1883, delivered to the Oregon Army Reserve in around 1890, and purchased by my friends father in 1950. When my friend moved to Oz he sold the rifle to me.
https://sportsdad60.smugmug.com/1873-Springfield-Trapdoor-45/n-gxR9F/My son shooting the Trapdoor.