Knowing the culture you are trading with (having history, psychology, xenology, language skills for a species/culture) probably makes a big difference. It is very easy for the uneducated and the ignorant to offend in foreign lands. Liaison looks like a really sweet skill for Traders. So, it is knowing that in some parts of the world, you haggle for items because they *expect* that and if you don't, you are not seen to value the item. In other places, if you haggle, you insult the seller who is only charging you a fair price. You have to know the difference to be a good trader, and that involves education.
But these sorts of situations can make excellent trade-game scenarios.
Also, you can add to that the customs of Free Traders - they cross the stars like our sailors cross the seas, and we have traditions for crossing various Tropics of Latitude and various Meridians of Longitude - the international dateline, the equator, tropic of cancer, tropic of ??? (I forget the other one). Plus there are traditions on doing certain things, like sailing around the Cape of Horn or crossing through certain canals or narrow passages. There is no reason free traders would not have such traditions, and I'd include among them:
First jump
After having engaged a pirate
First landfall
Crossing a rift
Perhaps crossing a sector or subsector boundary
First misjump (if you survive)
Recieving your first bonus (something tells me buying the rest of the crew drinks falls in here)
The end of a loop (arriving back at your home port)(first loop might have an extra special meaning)
Comissioning a new ship
Taking a new ship out the first time
Jumping a new ship the first time
Getting a promotion
These are all the sorts of things which may result in some form of ceremony.
Just as added trader trivia, I'll post a link to something I wrote, some slang and professional attitudes....
Slang
Professional and Cultural Attitudes