I have been in the industry I work in for 30 years (music instruments), so most of my sourcing for items related to this category has just come out of searching for something when the need arises for a local customer. The stuff I have listed that aren't music instrument related are just things I have come across that it turned out are worth listing after doing a little looking around. My music instrument parts I sell that are new, I actually buy in bulk and have made for me by machine shops to my specifications, again, from my experience in the music instruments field. I would suggest you find a category to use as your main category of concentration, one that you are fairly familiar with, and that you feel you can put your attention toward without getting bored with it. Don't buy in crazy numbers until you have something that you have seen you can sell. Also, related to that, some times of the year are more slow than others, I'm experiencing that right now after June and July being fairly busy.... just remember that there WILL be slow times, and don't freak out about it too badly when it happens... BUT, and this is important, DO research what other sellers in your categories are selling similar items for. I have my own brand for guitar parts that I push now, and the other site I sell on, which is only for music instrument related stuff (Reverb) has actually recognized me as a manufacturer, so when I list something, my own name is in the drop down for brands, which has added to my cred as a seller a good bit. I have a couple of stores and a couple of guitar builders and repair shops that buy from me on a fairly regular basis now as a result.
I have been here on eBay for 20 years now, I joined in 1999, and there are sellers here that have been here longer than I have. It definitely takes some time to learn the ins and outs of the site, and that's not even taking into consideration that eBay frequently makes changes to things.
Expect them to hold your funds until after a buyer has left you positive feedback until you get your feedback up to the magic number, which I think is 10, if I remember right. You WILL get your money as long as you don't have any problems with your transactions.
Lastly, if you are looking for a quick buck, this isn't the place for it. Sometimes that will happen, but most of us who have stuck it out for years here use this site as a supplement to whatever else we do in real life, which for me is repairing musical instruments for a living. I have a repair shop in my home, and I do repairs for local schools as well as for a couple stores in my area. And, if we ever get it off the ground, I'm also going to be the shop/setup/shipping guy for a new guitar company that's coming to my area. We have 900 guitars in a warehouse half an hour from my home, but we keep running into minor setbacks... hopefully this fall it will happen. I met with the owner last week, he's another guy like me with many "irons in the fire"... he installs sound systems in churches and schools, and is a licensed electrician, too, so he is constantly busy... keeping my fingers crossed.
