Multi-Location Inventory System Integration

I need a practical, reliable inventory system that keeps both of my business locations in sync while talking seamlessly to Amazon, Bol.com, and Back Market. The core workflow is simple:

• Each product must be stored with both an SKU and a serial number.
• The system must show purchase price and current selling price side-by-side.
• Stock levels at the two locations should be independent, yet I must be able to record transfers and instantly see the updated quantities at each site.
• Whenever stock changes—new delivery, transfer, or sale—the correct quantity needs to push automatically to all three marketplaces so listings stay accurate.

I am open to a custom build, a well-chosen SaaS platform, or an off-the-shelf package with strong API capabilities—as long as it meets the requirements above and can scale as we grow. Please include:

1. A brief outline of the technology or service you recommend.
2. How you will handle the marketplace APIs for Amazon, Bol.com, and Back Market.
3. A timeline for setup, data migration, testing, and go-live.
4. Post-launch support options.

Deliverables are a fully functioning, tested system linked to our seller accounts, complete documentation, and a short hand-over call so my team can begin day-to-day use without a hitch.