Photo and video copyright experts: where do old photos bought on eBay fall?

This is not necessarily about Steemit. Long before I'd heard of Steemit I started collecting interesting antique photos on eBay. I have snapshots, cabinet cards (these date back to the 19th century), and glass negatives (usually 100 years old or older.) They are of all sorts of subjects: people in Halloween costumes, Japanese Kabuki actors, people diving and doing other stunts on the beach wearing old fashioned bathing suits (from the 1920s-1940s), WWII sailors, children and their dogs, and just odd and unique looking people whose faces I wanted to remember. The glass negatives include Christmas scenes, antique textile patterns from the British Museum, rare photos of Native Americans, people riding old bicycles, etc.

Around 90% of these are candid, amateur photos taken by people of their family and friends. The other 10% are from professional studios or if they are not marked as such, clearly look to be the work of a professional photographer.

What I am wondering is whether I could use any of these images in something I create or if they are all copyright protected? If they are over 100 years old does that mean they are in the public domain? If they are not that old, but are backyard photos some family took of their dog can I use a photo of an especially adorable dog to create a t-shirt for sale? Could I use some of the other ones in a collage on Steemit to share my love of old photos, or would the fact we get paid for our posts make this a commercial enterprise and therefore off limits?

Should I simply err on the side of caution and not use any of them for anything public?

I'd love hear from people who understand copyright law much better than I do! Thanks in advance.