I used to sell a lot on eBay.  25 years ago I bought a typewriter shop at an auction for $1.  That started me selling on eBay.  Then for various reasons over the years I slowed down.   A couple friends sold a lot on eBay.  Both died in the last few years and I kind of lost interest. Facebook Marketplace is the current hot spot but it has its own challenges. Here's a few eBay tips from my and their experience.  I'm probably going to remember other things as we go so this may be a bit disjointed or edited:

Be sure you understand fees. There's listing and final sales fees.  If you have a store they are different than the occasional seller.  And different categories have different fees.  I don't hardly mess with anything under $10-15 these days

#1 is understand shipping.  It will make or break you.  Before listing get weight and size and estimate shipping cost.  My preference is "free" shipping (you build it into the price) or flat rate regardless of where it's going.  Compare shipping. Weight and size make a difference. Do not just go to the Post Office.  Ebay and Pirateship have negotiated rates with USPS that beat it. Sometimes UPS or FedEx beats USPS  Be sure to factor in pickup fees or the cost of driving to a drop off.

I use Pirateship a lot for things not through eBay.  Rates are same or close.  And you print the label including postage.

I once sold a CB antenna to a guy in the east somewhere.  FedEx beat everyone for that combination of weight and size.  And for midwest buyers I've even used Speedee Delivery like Shoup uses.  Fastenal used to do shipping as well.   I sold planter boxes to a farmer in ... Kansas I think. I shipped them through Fastenal.  One of my friends shipped tractor tires through Fastenal. 

Speaking of where it's going, I'd specify ONLY US until you get experience.  Solves issues before they start.  One of the friends that died sold printed shirts worldwide.  After some things changed a couple years ago he only shipped to US addresses.

If you have a part number include it, in the title if room.  Many times I've been working on something, say a semi has a busted hood latch.  It has a part number.  Search eBay for that number, buy it and go back to work.  Always include part numbers if possible.

I'll probably think of more and come back  Gotta go for now

edit:  Thinking about this is bringing back memories.  My good friend Danny made his living from eBay. Over 95% of his sales were eBay.  He taught me a lot more than I'm relating here.  At one time he was one of the largest T-shirt sellers on eBay, a "Power Seller".  If you bought it today he intended to print it, package it, and ship it by 2 PM tomorrow.  (That's when USPS picked it up at his shop right before picking up outgoing mail at the Post Office.)  On a slow year he'd spend $15,000 on postage.

All that from an old grocery store building in little West Union, IL

One of the changes since I started is auctions.   Back then everything sold at auction. Buyers would mail a check.  When you got it you waited for it to clear ad then shipped.   Nobody is that patient today.  They buy it now and want it yesterday.    Auctions still work, but rarely.

Ebay has been hugely influenced by Amazon.  Buy it now and want it tomorrow with free shipping and returns for any reason are all caused by Amazon. If I were to get back into selling on eBay very seriously everything would be packed and ready to ship before it was listed.
Enough reminiscing.  Feel free to drop me an email if I can help