I've spent a lot of years on Ebay, almost exclusively as a buyer.  I've bought a very wide range of items from a variety of sellers- from adding to my various collections to purchasing antiques, decorations, clothing, music, toys- you name it, I've got one or three.  During my travels I've seen a lot of consistent behaviors from buyers which have either kept me from returning as a customer or prevented me from making a purchase in the first place.  So in the best interests of both buyers and sellers, I have compiled a brief list of those things that turn me off as a customer.

First, the practical advice- or, how to get me to consider a purchase.

Describe your item:

 Use simple, plain english, and tell me what I need to know.  Don't use all caps or sentence fragments.  Good spelling, punctuation, and grammar are a must- if I can't read what you've written, I can't take a chance on you. Likewise, don't clutter the auction page with paragraphs of irrelevant information, blinking graphics, and scrolling text.  Ditto the twinkly cursor, the floating fairies, and the background music.  I want to see what you're selling; everything that's not relevant to that is a distraction and a browser killer.

 Be honest. Tell me what it's flaws are, what you estimate it's worth to be (if that's not apparent). Do: Be thorough. If clothing, give the size AND the measurements.  Use pictures of the actual item whenever possible, including close-ups and images of any damage.

Do: give the closest shipping estimate you can.  I've left many, many items behind because I could not find the shipping cost.  Don't assume I'm going to write you to ask, either- I may do that for a special item, but most times I'll move on to a seller with a better description.

Honesty:

It's better to be honest and make a little less money than to lose a customer or risk negative feedback. 

Disclose everything you know. A chip or stain or ding probably won't prevent me from purchasing an item, but an undisclosed flaw is a deal-breaker- I won't come back if I feel deceived.  If you smoke or keep pets, say so- don't spritz with Febreeze and think I won't notice- I will!

Don't  weasel.  For example, if your collectible is a reproduction, or there is a reasonable doubt, say so.  Don't use weasel words like "appears genuine," or "looks unused".  Don't say "vintage" if you mean "vintage style."  If you're not sure, say so- and provide good, clear photos. Likewise, If your photo is of a representative item, say so- never assume 'similar' is good enough.  Likewise, if your photo is representative, print that right on the photo or disclose this in a prominent place.

If you make a mistake, own up to it.  If a customer must return something because you made an error, pay the return shipping- and refund their shipping charges as well as their purchase price.  Never, ever expect a customer to pay for your mistake.  Think of the future: you might lose profit making right on a mistake, but that's always better than losing a customer.  If you're good to me, I'll come back.

Have an honest shipping policy:

• Be reasonable- you should not expect to make a profit on shipping- don't charge me for shipping materials if you use newspaper and recycled Amazon boxes, and don't charge 'fees' intended to increase your profit rather than cover actual expenses.
• Don't lowball the minimum bid and try to recoup it in the shipping charge. I won't buy a .99 cent item with a $10 shipping charge because it is inherently dishonest. 
• Lose the 'handling' fee- if you have expenses for packaging, put them in your minimum bid amount for the item.  I DO look at the shipping price, and that extra five dollars DOES matter.  If you accidentally overestimate shipping, refund the difference, without waiting to be asked.  It matters!  I've left many sellers without a second look over this discourtesy.  If I'm charged $12 for shipping and materials, and my item arrives in a recycled box with $3 postage on it, I'm going to resent it, even if I never say anything to you about it. 
• On a related note, don't overcharge on 'combined' shipping.  A reduced charge is not a bargain if the total exceeds the actual postage.  A  seller's $1 per item 'additional' shipping charge once resulted in a thirty dollar overcharge for me and a fraud investigation for her.

Feedback is important:

Believe it or not, I DO read your feedback. To the sellers who believe their 98.1% rating is a positive- it is not.   Hundreds of "A+, perfect seller" ratings mean nothing to me if accompanied by three buyers complaining of deception, mistreatment, rudeness, or lack of communication.  More than one complaint about the same behavior is a red flag- I won't even consider you. 

 If people continually complain about the same thing, address the behavior- sixteen different excuses for late shipping are a waste of time, but if all of your negatives are well in the past I'll assume you learned your lesson and I'll forgive you.

Admit your mistakes: A negative comment followed with an apology or an  honest explanation impresses me a lot more than angry splutter, nasty comments/insults, or sorry excuses.  Likewise, don't get personal- I won't buy from a seller who makes derogatory comments about customers, period. 

Likewise, sellers who attempt to embarrass customers over their purchases- if you sell items where customers expect discretion, then be discrete.  This almost goes without saying, but I can't tell you how many times I've seen retaliatory feedback along the lines of "enjoy your ***, pervert."  It's heinous, and a sign of a major character flaw...and I wouldn't buy from you if you were the last seller on earth.

Don't leave retaliatory feedback. If a buyer is unhappy, and you weren't able to resolve an issue, they have a right to present their complaint.  If your only grievance is that they complained, address it with the buyer privately.  Before you rush to get revenge on the person who complained, consider the consequence.  I've avoided many sellers simply because they seemed to feel the need to answer legitimate complaints with nasty, negative feedback.  So don't compound your error- accept your lumps and move on.  If the feedback is unfair, don't sweat it, as it will be obvious- but a response in kind just looks petty- just leave it and trust me to know the difference between an honest complaint and a crank.

Be the bigger person.  If a buyer makes their payments and responds to communications in a timely fashion, give them positive feedback.  Do not insist they 'gift' you with feedback first, and don't solicit feedback if you can't manage this simple courtesy.  If you're afraid of negative feedback, then you are not conducting yourself honestly, and you need to correct your behavior rather than extort your buyers!  It should also go without saying that a seller who writes in an auction "I will give feedback when I get feedback" looks very suspicious, and again, I will avoid them- feedback is what you give a buyer for being a good customer, not something you hold over their head to get what you want.

How to get me to come back:

Ship on time:

Make your shipments on time, especially if you are charging for premium postage.  While I'll accept a couple week's wait from a casual seller, I won't accept delays from a business.  Be professional, and do it on time.  Nothing is a bigger turnoff than "I only ship on Fridays," or having my priority package shipped a week late. 

If you have an unavoidable delay, let your buyers know about it.  Don't be that guy who expects payment within two days and then waits a week to amble over to the post office. 

Plan ahead for emergencies, too- have a backup plan so that if you break your leg, someone can make your package run.  Some emergencies are unavoidable- natural disasters, etc., but personal issues are just that.

Don't delay time-sensitive items- either get it out on time or refund my money promptly.  Don't send me my Christmas ornaments in January or my Halloween costume in November unless you make it absolutely clear at the time of purchase that it won't be there on time.  I know you can't rule the post office, but do please make timely shipments!

Communicate!

And, most importantly of all: Communicate!  Answer your email.    A sale may not depend on the size of the widget, but I do know that if my question is not worth your time, you aren't worth my money, either.  Chances are, if you won't communicate before a sale, you're not going to when there's a problem later.