Whatever you are selling, someone will have sold one before. So always do some research before making your listing. Use the same search term (e.g. Tiffany bracelet) then choose the "sold items" from the left hand panel. Look through the listings and try to work out why some sold cheap and some sold expensive, and which one yours is like.
You can put a reserve price on an auction listing, but this usually has an extra fee. Starting low can whip up interest, but there is still some risk that you will get a disappointing price unless you can gauge the popularity from past listings
You can put a "Buy it Now" price with an auction (e.g. higher than you'd expect the auction to reach) but it disappears as soon as someone makes a bid.
You can combine a Buy it Now price with a "Make an Offer" option. By default eBay passes on any offer more than 50% of your BiN price but you can adjust that. If someone makes an offer you get two rounds of haggling to get the price up, then you have to decide if you have reached an acceptable price or not. If not - just decline.
If you have done your research, you will know whether the offer you received is good or not. Some people try to con you by hoping that you don't realise what it's really worth, others offer a good price because they don't want to risk losing it in the auction.
Always complete the sale through eBay and pay the fees, or you will lose your account. eBay messaging doesn't allow phone numbers or email addresses that facilitate completing a sale off-eBay.