Online dealings are easy, just so long as both parties respect each another.
I've been buying and selling for over a year now, and here are some of the things I have gathered from experience.
For the sellers:
1. Be reasonable in pricing. Do not inflate the price out of greediness.
2. ALWAYS post actual pictures. Use high-definition camera and good lighting.
3. Describe the item in detail. Provide proof of authenticity if necessary.
4. Don't make the buyer wait more than 12 hours for a response.
5. Be on time for meetups. Let the buyer know what you are wearing.
6. Smile and thank the client. Don't just take their money, then leave.
For the buyers:
1. If you do not have money, do not bother inquiring about the item. It's a total waste of time.
2. Ask politely for a small discount. DO NOT HAGGLE like there's no tomorrow.
3. Read descriptions properly. Do not ask about something already posted in the ad.
4. Schedule for a meetup only when you are a SURE BUYER.
5. Be on time for meetups. Being a customer doesn't give you the right to be late.
6. If you decided not to buy the item, let the seller know. Don't just disappear into thin air.
7. After the transaction, provide honest feedback to the seller.