I am genuinely sorry for the issues you've been having. Please hear me when I say this: It isn't because you suck. I was looking through your listings and I saw what @reallynicestamps did. I'm a dude, but I understand what they are saying, though that was not my take. Instead, when I read the description, I thought to myself, this is someone who loves selling and what they do. This is someone who cares about their customers, even the bad ones. It does not at all sound like the person who responded to some of the negative feedback or in some of their responses on this thread.
One thing I noticed is that in your descriptions say you take returns back no questions asked. Your settings for the listing have it set up that you don't accept returns. That right there opens you up to having problems. My opinion is that you need to accept 30-day day returns. Whatever you decide, make sure what you say in the description is consistent with how you have your return settings set up, especially if you set the listing up to not have returns. There are people who purposefully seek out these type of listings, knowing there will be issues trying to return the item, so this way they get their money back and keep the item.
I don't know your financial situation and you truly may not be able to close up for a week. I do think stepping away briefly would help you just to refocus. You've got a good business going. I looked at your sales, so I can see you are making money. Don't throw it all away. I don't think you are the problem, but I think your attitude is the problem. Ask yourself what happened to the person who wrote, "I stand behind every small auto part I sell," and "Our return policy is straightforward and customer-friendly." I may be wrong, but I doubt that seller was having these issues or, if they did, they handled them better.
Bad customers are part off reselling. I hate it. I wish it were not so and I wish eBay made public things like how many returns a buyer has, for example. I don't think something like that would drive good buyers away, but it might scare off some bad ones. I've gotten much, much better at picking out potential problem buyers and blocking them before they can buy. Making sure I have clear photos that show items from every angle helps reduce issues. Making sure the item condition is clearly written, and not just defaulting to eBay's definition of "used" and then repeating that in the description, which most people don't read anyway.
As far as lost packages, are you using a thermal printer to print your labels? If so, make sure the bar codes are clearly printing so they can be scanned.
