1.Set your business up to accept as many forms of payment as you can. PayPal, personal checks, cashiers checks, both domestic and International money orders, International bank transfers, etc. Every additional payment option you add will increase your customer base.
2.Don't exclude buyers from foreign countries or with free email accounts like Hotmail or AOL. Ultimately, you are only hurting yourself. In some categories best buyers are located in a different country than you are.
3.Keep your ads simple. If you confuse buyers, they will either by-pass your offerings, or leave you negative feedback because they read something into your ad that you never meant to be there.
4.Don't attempt to increase your profits by a few dollars by charging excessive postage. Buyers are not stupid. They realize what you are trying to do and most will avoid your auctions like plague.
5.Give your customers all facts to bid intelligently. Don't tell them you will figure out what postage will be after auction ends. Tell them in your ad what class postage you use and what actual amount will be.
6.Don't discriminate against buyers who don't have a perfect feedback record. As long as you have good business procedures in place (such as waiting an appropriate period of time for checks to clear), there is no need to look downward at buyers whose reputation might have been damaged just because they got a bad deal. It happens to everyone and someday it will happen to you.
7.Don't discriminate against buyers who have little or no feedback. Remember, you were in same position at one time. Once you have been in business for a while you'll learn that a buyer with little or no feedback is no more likely to screw you than one with a shooting star.
8.Don't make customers jump through hoops in order to buy from you. Just because you had a bad experience with one buyer, that doesn't mean you should threaten everyone else in advance. Doing so won't hurt your customers, but it will hurt your business.