Small e-commerce sellers spend 4-6 hours per week manually syncing inventory across Amazon, Shopify, Etsy, and eBay

You sell handmade candles on Amazon, Shopify, Etsy, and eBay. You have 80 SKUs. When you sell 3 units of 'Lavender Dreams 8oz' on Amazon, you must manually update the quantity on Shopify, Etsy, and eBay — or risk overselling. You sold the last unit on Amazon at 2am but Etsy still shows 1 in stock, and someone buys it at 8am. Now you must cancel the Etsy order, get a defect on your seller account, and apologize to the customer. This happens 2-3 times per month. You spend 4-6 hours per week manually updating inventory counts across platforms. So what? There are 2.5M+ third-party sellers on Amazon US alone, and most serious sellers list on 2-4 platforms. Multi-channel inventory sync tools exist (Sellbrite, ChannelAdvisor, Linnworks) but cost $100-500/month — more than many small sellers earn in profit per month. Free tools have 15-30 minute sync delays, which is enough for overselling during busy periods. The cheapest reliable solution is manual updates. An agent that could monitor sales webhooks from all platforms in real time and instantly update inventory across all channels would solve this for $20-30/month. Why doesn't this agent exist as a cheap solution? Each marketplace has a different API (Amazon SP-API, Shopify Admin API, Etsy Open API, eBay Trading API) with different rate limits, authentication methods, and inventory update latency. Building and maintaining 4 API integrations is expensive. Existing tools charge $100-500/month because the API maintenance cost is real. Nobody has built a lightweight agent that monitors webhooks and pushes inventory updates without the full-featured (and expensive) channel management suite.