Short answer:
A) You are correct to be upset.
B) They have the right to sell at any price and upset you.
Present the situation as a hypothetical next time.
I've had the same experience. There is no rule for this, except explicitly by clan. There is an unwritten voluntary etiquette that a discount should be given to others, but don't take strong offense if they don't. Some friends always keep the bigger half when they share a sandwich. You're still better off in the game keeping them as friends, if they invite you to boss battles.
Just silently remember what kind of friend they are. The next time you get a drop they want, stick them with the small end of the sandwich.
1. This post is pointless
2. So what if he tanked? Drop chance would be the same anyways
3. To be honest, a lvl 142 is much more help than a lvl 103
4. Naming and Shaming is against the forum rules,but since our board is almost unmoderated
Points 2 and 4 from Regenleif are good advice for everyone, making point 1 false.