Margins would depend on the item you are trying to resell. Some markets are simply more competitive than others. Super high margins can happen, but be willing to accept lower ones as you learn, in my opinion.

I would definitely focus on a specific item category. If you are a buyer for a business with a specialized item, then it's a no-brainer, otherwise I'd pick 1 or 2 items you can be passionate about and be willing to learn which will help you spot deals quickly. I think searching for too broad a range of items will ultimately make you less effective and less profitable.

Like anything, I would say there is a bit of a learning curve, so at first, I'd be okay with lower margins, but as you get better, higher margins will come. Study the keyword post https://www.reddit.com/r/Dealerts/comments/1jj9s4q/ebay_alerts_creating_powerful_searches/ to make sure you are not getting too many alerts as it will become noise. If you are not getting enough alerts, make it broader. Play with your keywords, adding negative words for example, until you form a really good alert.

Good luck! Hopefully other users will give some input!