- Yes, by modifying the existing products module. Check details here (Inventory Module)
- If you want to use inside CRM, needs custom module to be developed. (can do via studio)
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- Same as 2
- Check how Web to Lead works. Wordpress does provide hooks which posts content to any other URL for notification or processing.
- I would suggest just keep payment records and do the payments on the vendors site. Just a link to connect to their site.
- Use workflows to create alerts, otherwise a dashlet with status filter is enough.