As the title suggests, these are just guidelines. So, don't take this thread to mean this is gospel, just some ideas to help you sell more and for buyers to not get taken.
1. Everyone - Golden Rule
Treat others as you want to be treated. If you are selling a service, then make sure you complete said service in a reasonable time frame and keep in contact with your clients so they know when to expect the finished product.
As a buyer, pay in a reasonable time frame. Nothing is more frustrating to a seller than someone committing to buy and not paying for it when payment is due. Also, as a buyer you owe it to the seller to let them know if your finances have changed and you can not complete the transaction.
2. Buyers - Caveat Emptor
Buyer beware. If something sounds too good to be true, it probably is. So, ask clarifying questions.
What kind of traffic?
How much traffic?
How old is the domain?
Do you have samples?
Check their iTrader. See what other posts they have made, talk to past customers (if they have any). If you're still not satisfied, pass on it. There will always be other "can't miss" deals.
3. Sellers - Disclose other sales
If you are selling the same service on SitePoint, DigitalPoint, or whereever, let people know. You don't need to post a link to the thread, but at least disclose where you are selling. There have been problems with recent sellers putting customers from another forum over their customers from WickedFire. This is intolerable. A customer is a customer, regardless of where they purchased from you.
4. Sellers - Post the type of sale it is
Is it an auction? Buy it now (bin)? Offers? What? Please clarify your terms up front. If you are selling auction style, it would be best to post the ending date, and include a time.
Offers should have a minimum acceptable offer (just a suggestion, not required), unless you really don't care what kind of offers you get. Often I've seen "Make offer" and then a potential buyer posts "I offer $50" and the seller gets offended at such a low offer. If you think your item is worth $500 and don't want to deal with low offers, then post your minimum acceptable offer.
5. Sellers - Payments
Specify types of payments you accept, and the time frame you expect to get paid in. For example "I accept PayPal, Moneybookers, and eGold. Payment must be made within 3 days of [offer acceptance, auction end, whatever]" Also, it is advisable to add what will happen if payment is not made in that time frame "If payment is not received within 3 days, negative iTrader feedback will be left and item will be relisted"
6. Sellers - New Members
While it is not a rule (yet) that you can not sign up and start selling right away, expect to get a lot of flack for it. People here want to do business with trusted members of the community. To become a trusted member, you need to participate in threads outside of this section. Create threads with useful information, and generally prove you are someone that people want to give their hard earned money to.
If it is suspected that you are just a scammer / spammer, you will be banned without warning and will NOT be unbanned. So, be very careful if you are a n00b and want to post in this section.
7. Sellers - PM Spam
Don't do it. If you are selling a product / service and you decide to use the Private Message system as you own personal advertising service, you will be banned forever, no questions, no warnings.
10. Sellers - Bumping Threads
Bumps are annoying and rude. "Bump, bump, bump" or "bumpity bump" or even just "bump" should be avoided. If you have a service / product for sale and people are not responding, do not just bump your thread to the top. If people are interested, they will do it for you by responding to your thread.
These are just guidelines, not rules, but if you follow them, I can assure you that your selling / buying experience will be more profitable and fun.