I wish you the best of luck on your passion project. It sounds like something I would like. I basically played solo for my entire time at RuneScape (1) and FFXIV unless forced not to. Coming up with a viable model for your prospective game's economy will be challenging. It brings to mind when they implemented the Grand Exchange feature into RuneScape. I think that's like the auction house idea you mentioned. That's what I imagine when I think of a solo RPG with an MMO economy.