As a business seller if I listed 7,600 items like the OP it would cost me best part of £500 per month in listing fees alone not to mention FVF's which would add many more £100's. Returns that a business seller is obliged to offer also cost 

The OP is looking for advice.... Maybe the it's time to start trading legally, paying your fees and start accepting returns like the majority of honest law abiding business sellers and you might find you will stop getting "unpleasant messages from eBay" !