1. What sells best?
Anything and everything, things you think no one would be interested in will surprise you.
2. If you’re selling something new and sealed but unused do you still have to mark the price down quite a bit?
It depends. Is the item still being sold, then mark down slightly. If the item isn't being sold anymore it could actually be worth more than the original price. Research is key.
3. Any do’s or don’ts? Mainly dont’s would be good
Don't sell as an auction, always use the buy it now option. My second don't, is don't follow the estimated postage that Ebay suggests.
Also regarding postage, always choose the tracked option. Most Ebayers aren't scammers, but treat them like they are and you won't get scammed.