Omnivore: order management across marketplaces. Strong on Australasia (TradeMe, eBay AU). DataFeedWatch: feed transformation. Strong on Western marketplaces, complex attribute mapping.
Brief is “get our products listed” ? DataFeedWatch. Brief is “manage inventory and orders across channels” ? Omnivore. Both ? run together with clear ownership.
I do not build custom marketplace integrations from scratch. Maintenance is brutal. Use tools that have teams maintaining feed-spec changes for you.