Losing buy box eligibility for the majority of your FBM (Fulfilled by Merchant) listings can indeed be frustrating, especially if your account health is otherwise in good condition. While it's unfortunate that you haven't received satisfactory assistance from seller support, there are several steps you can take to improve your buy box eligibility:

Check Your Performance Metrics: Review your seller metrics to identify any areas that may be affecting your buy box eligibility. This includes metrics such as Order Defect Rate (ODR), Late Shipment Rate (LSR), and Valid Tracking Rate (VTR). Even if your overall account health is good, specific metrics may still be causing issues.

Optimize Pricing: Ensure that your prices are competitive compared to other sellers offering similar products. The buy box algorithm heavily considers price, so adjusting your pricing strategy may improve your chances of winning the buy box.

Maintain Sufficient Inventory Levels: Make sure you have enough inventory available to fulfill orders promptly. Running out of stock or having long processing times can negatively impact your buy box eligibility.

Improve Shipping Performance: Ship orders promptly and provide accurate tracking information. Consider using faster shipping methods or optimizing your fulfillment process to reduce delivery times.

Enhance Product Listings: Improve the quality of your product listings by including clear and accurate descriptions, high-quality images, and relevant keywords. This can help improve your visibility and attractiveness to customers.

Respond to Customer Inquiries Promptly: Be proactive in addressing customer inquiries and issues to maintain high levels of customer satisfaction. Prompt responses and excellent customer service can positively impact your seller metrics.

Monitor Competitor Activity: Keep an eye on your competitors' pricing, inventory levels, and performance metrics. Understanding your competition can help you adjust your strategy to stay competitive.

Consider Fulfillment by Amazon (FBA): If feasible, consider using Amazon's FBA program instead of FBM. FBA listings often have higher buy box eligibility due to Amazon's fulfillment and shipping capabilities.

If you've already tried these steps and are still experiencing issues, you may want to consider reaching out to seller support again or seeking assistance from Amazon's Seller Forums or professional Amazon seller communities. Additionally, you could consider consulting with a professional Amazon seller consultant or expert for personalized advice tailored to your specific situation.