This isn't something I've come across before as an extension. Do you have any thoughts on how you'd like to give product information to resellers and how you'd like to receive orders from them?
Product export isn't too bad because the simplest method would be to run a scheduled export and you could even use the FTP process to copy it on to a server of their choosing. Many of the marketplace extensions just send an email to the dropshipper. If you can accept orders like that, that's easiest. Receiving orders programmatically from resellers is a bit more difficult. In theory you would need to provide a magento extension (assuming all resellers were on magento?) that then submitted orders back to you. An extension on your store would then need to be able to cope with tracking reseller order numbers and you'd probably want to store where each order came from. This is very likely going to be custom work.
Another approach would be to ask resellers for a daily order spreadsheet and then you can write or use an extension to do order import from CSV https://marketplace.magento.com/catalogsearch/result/?q=order%20import#q=order%20import&idx=m2_cloud...
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