In principle, I have no problem with telling the players what source/s I'm pulling 3pp material from. In practice I often don't know what 3pp spell/s I'm going to place before I do so, so I'm not sure what the player/s would gain. Also, some of my sources include spells that I'd likely hard nope on, and I'd be unhappy to have a player get their hopes up about those. Also, it's not wildly unusual for me to edit 3pp material when I type it up for addition to the game (and once it's in the game, I'll make that available to the players).Again, this is fine. I’m certainly don’t want people to think I’m hardline about RAW. I’m not. I use all sorts of non-standard stuff. No problem.
But let me turn it around. Player asks you to see your spells. Even if he can’t use them, he just wants to see them. Maybe to draw inspiration. Maybe to check the math. Doesn’t matter.
Would you hold that no, the player absolutely may not look at your list of homebrew spells? How would you feel about a dm that flat out refuses?
As I said earlier, this just raises so many red flags.
But then, if a PC gets hit with an attack with a rider, I'll announce the rider--complete with DCs and further effects--when they get hit. I'm fine with the players knowing the mechanics for what's going on, and I in fact kinda rely on my players to keep track of things like that (they're not the sorts of players who, if I forget the rider, will let it go, even if it makes things worse for their characters).