Blayne —
1) Your users would not be registering on WordPress.com .. they would be registering on YOUR SITE ONLY.
2) The registration process can be made relatively quick and painless .. I just put up an Ultimate Auction WP site for my kids’ school fundraiser .. it’s working well. I would highly suggest the following additional plugin: https://mikevanwinkle.com/wordpress/how-to/custom-wordpress-registration-page/ ..
3) WordPress, when a logged in user is a requirement, redirects to the Login Page .. they must then click REGISTER which is below the user/pass boxes .. you may want to send users a START page LINK which is directly to the register page (or custom register page if you install above plugin) .. presumably you currently have NO users, so everyone needs to be registered, so sending EVERYONE a “Start Here” page would make sense.
4) I’m not sure of the Pro version features exactly, but I personally feel safer having it be a closed community of registered users bidding on our charity items. I believe that the UltAuc plugin used to be open for all to bid, but there was discussion in this forum to force registration, which, as I mentioned, I prefer.
Have a look at the plugin I mentioned above .. make sure you