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