Selling a pricey personal item always becomes too emotional, I find. People have a lot invested, both monetarily and emotionally, and are often incapable of thinking either clearly or logically, when in the sales process.
This can translate to heated "discussions" between sellers and potential buyers, which are almost always non-productive.
The biggest challenge for both buyers and sellers is to be able to remain calm and refrain from using an insulting tone or hurtful language.
As someone pointed out in an earlier post, it's a good idea to have your "absolute max" (or min) in mind before you begin bargaining. Then, you need to decide if you are going to go straight to your "secret" price right away and just stubbornly stick there, or do a "stair-step" type of negotiation, where you give, the other party gives, and so on. Both methods are effective, I find.
If you find the other party is getting under your collar (upsetting you, in other words), try imagining them in a silly light to get yourself back under control. For example, if the other party is a large male, imagine that they are right now wearing a ballet tutu (that frilly little dress ballerinas wear), and you will quickly feel yourself becoming more at ease.
The only other things to remember are to be patient, and remember that it may take some time for the other party to really believe that you are not going to go where they want you to on price, or for them to accept buying/selling at that price.
It needn't be a negative experience, if you have your emotions firmly under control.
Lastly, remember this old truism: unless you are willing to walk away from a deal
, you are not negotiating
. So be as firm with yourself as you are with the other party.
Sometimes you don't buy a particular item on the first go-around. Don't think for a minute that that means you never will have it. Many sellers (and buyers) can change their mind, given a little time, so always leave the door open if you don't make the deal you want, so that the other party is comfortable to contact you again, should they have a change of heart. You might change your mind, too!
So good luck!