Currently if you implement a new marketplace that is not Amazon or ebay you have to manually increase stock level on any item you want to sync to push a sync through.  (You fail to mention this in your documentation) 

This is a work around and not how it should be. This needs sorting. 

Fix this and implement a proper method and document this so that time is not wasted with support trying to figure out why stock is not syncing. 

All you need is a button on the channel integration on the specific channel settings that says, Send full feed or something like this which sends the complete feed to the marketplace. 

This is not a feature, its basic functionality.

As a developer of an app that is used by multiple sellers, it frustrating having to get them to jump through hoops just to do an initial sync of their stock levels.

I agree that an "Initial Sync" is basic functionality that should be a standard feature for any channel integration. 

I appreciate a time limit between triggering full syncs would be necessary to avoid abuse.

Need to get a force sync on the integrations screen. 

I hope more people see this suggestion, you will want it when you come to integrate a new marketplace. 

Putting false stock values in just to get it to sync seems ridiculous and functionality is clearly missing here