I sell a lot on Ebay UK and rarely encounter problems. Mostly WWII - Vietnam-era clothing, helmets, patches etc, $5 up to $500+, some very rare and expensive+ . I have sold certainly over a hundred items this last year and have a few guidelines I would think might help;
1/Don't use only Buy-it-now for this kind of stuff (items just sit there forever - seems to put people off). I occasionally add a BIN option to an auction (the BIN option will disappear after the first bid) and sometimes the final bid is actually higher than the optional BIN price.
2/ Don't be afraid to start low. I try and list everything with a low start price. If you have a good, thorough listing and good photos you will not be disappointed. I've sold six or seven repro A2s over the last year or two with low starts (like 20-50 GBP). You'll accrue watchers as fast as you can count doing this and my last disappointing final price was in 2008
3/ Put effort into good photos. (should be rule number 1 really) You don't need a great camera, I use my iphone 8. You don't need studio backgrounds, I use my Victorian wooden floor or hang jackets on a wooden hanger on the back of a wooden door. I use white paper or light brown card for patches. Get all your household crap and body parts out of shot and use good natural light. Crap photos are a total waste of time.
4/ Ask Zero feedback bidders to contact you before bidding. I had a rash of non-paying/idiot bidders in 2018 and they all had zero feedback. I added a line at the end of my listing that zero feedback bidders must contact me or their listings will be cancelled. this small measure has absolutely worked but I have cancelled a few bids. It's the only warning I have on listings (don't scare people off with lots of crap about who's responsible if items get lost in transit-if it gets lost the buyer gets a refund, ebay makes sure of that).
5/ Use your blocked bidder list. This is where you add sellers who you want to block (I can never find it on ebay's site, it's deep in there somewhere, so I google it and get the link every time!). They then can't bid and can't contact you, hooray! In my early days I needed to add a few. One guy sticks in my mind - his item (an M1 helmet headband) didn't arrive in France (from the UK) in 2 days so he started deluging me with complaints. I've only blocked a few bidders and none recently but it's worth it.
6/ Be polite and use good English on your listings. Don't give potential buyers a reason to think you're an idiot and avoid you.
7/ Charge actual postage and do your pricing research. There are some quite fantastical postage charges on listings sometimes ($24 postage for a $9 WWII patch to the UK?!) If you can't be bothered checking for real, safe postage prices then just don't expect to succeed on ebay. I use Royal Mail Price Finder for all my listings. I list with tracking/signature when required (over around 30GBP) and Special Delivery over around 100GBP. Cheaper things I pop in an envelope and I have had zero lost items in the last 10 years.
8/ Use Templates. When you've figured out postage and general listing features for a particular type of item save a listing template so you don't have to reinvent the wheel.
9/ Post Worldwide. I post worldwide but sadly have a few exceptions (China, Indonesia, Ukraine) due mainly to the fact that all the zero-feedback non-payers I had came from the first two and the Ukraine is a postage nightmare (there's a war going on). Not posting Worldwide is a great way of limiting your sales! It's 2020, it's a global village. post Worldwide, it's seldom a problem and yes, it takes 10 mins to look up accurate postage rates but then it's done and you can set a template.
I may have missed some things but hope that helps!