I am an eBay seller. I do it part time, but run it like a business, since that's what it is. I sell mostly vintage clothing with a few contemporary men's items if they're in high demand. I also sell vintage kitsch, if it's in great condition. I love hooking up a buyer with something he loves and has been searching for.
I ship using all new materials that are branded with the eBay logo and my store name. Everything is packed neatly and has been freshly washed. I try to make it a delight for the buyer to open the package. The anticipation is part of the pleasure. Plus, you never know when your item might end up in someone's YouTube unboxing video!
Buy It Now is pretty much the way to go for almost everything unless it's something that you know will be in high demand and you generate interest for it on Instagram. Great photos, lots of details, and a 30 day return period are what will sell your item. When I buy on eBay, I shy away from sellers who don't take returns and who fill their listings with multiple paragraphs about what they will and won't tolerate from a buyer, and excuses about how they can't be expected to catch every flaw in an item, etc.
When it comes to pricing, it's crucial to know the market. If I have something special, I put a high BIN price on it with Best Offer and generally will throw in free USPS Priority shipping. I consider a lowball offer to be a negotiating tactic and love to work deals with buyers.
I keep around 500 items listed at any one time and am always on the lookout for cool stuff. My inventory is stored in a climate controlled storage unit. Clothing is on zracks, fragile items on shelving, and everything else in matching, numbered bins.
I am fortunate to live in an area where nice stuff is inexpensive to find and there is very little competition for it from other resellers and other people looking to buy for themselves.
I spend about 15 hours a week on eBay stuff, prepping items, taking photos, doing measurements, getting shipments ready, etc. I netted a little over $12K last year. That's chump change to a lot of people, but it's decent money for doing something fun.