In answer to your questions:
1. No neither of these are necessary for use in order to still allow secure transactions, and are entirely optional. You can read tutorials about both of these
here and
here which explains their functionality a little more.
2. We previously used a plugin for this functionality which allowed users to upload their image, but maintaining compatibility between the theme and the plugin was problematic. Once we started using Gravatar services it became a lot more efficient.
3. There is no default functionality which limits ad posting, and memberships allow unlimited posting within a pre-determined time frame/price. Membership pricing options are not mandatory so if you'd rather charge per ad only, you can.
4. If the featured ad option is charged and the transaction is completed then it will publish automatically. If it's free, with moderation required for posting, then yes you would need to manually approve it.
There are no dumb questions, we're happy for you to clarify these things before you decide on your purchase. Thanks.