This is a very good question as prevention is better than cure; more so categorically applicable in legal domain. Nowadays, consumer courts are very active and I have filed several complaints related to deficiency in services in the consumer forum where the terms and conditions of the service providers were not clearly defined.
Keeping track of your question, the right approach is don't promise what you cannot deliver. For instance, in a recent consumer court judgment, the car manufacturer delivered the car around 15 days late and the husband could not present it to his wife, resulting in deficiency of services. The car dealer was awarded with heavy penalty by the consumer forum.
For online selling you would have to ensure the following:
1. all charges including ticket charges, service, other taxes should be mentioned clearly;
2. in case if the tickets that you would be selling has certain terms and conditions from the airline, you have to clearly mention it, and ensure that the customer accepts it prior to going ahead with purchase;
3. clearly mention disclaimers and your liability on the site; albeit not out of fear psychosis but what is true, correct and factual;
4. do not have any hidden charges;
5. general ticketing procedure like checking in on website to ensure the reservation on the flight need to be enumerated, though most of the passengers already know about it;
6. do not assume anything, as based on my personal experience, whenever any company assumes anything no matter how big or small or obvious the situation might be; that's what lands them in grave trouble;
7. believe and implement the philosophy "stitch in time save nine";
8. last but not the least, do not provide false information or misrepresent information to the customer;
9. finally, it would have been better if I could have had the terms and conditions to ascertain that what you have with you is legally compliant.