Honestly I have to agree with you that I think it's a bit of a dirty trick to do something like that. They may have had all the best intentions, but they still cheated the system you had set up.
The only thing you can really do is be wiser in the future. Set up your rules before you get started and take a tough stance against behavior you don't think is fair--be sure to account for this kind of thing. Make it clear that people who don't play by your rules will be kicked and not asked to return.
The group I usually do runs with has a general rule of "don't lot if you don't really need it", meaning don't even roll if it's not going to go from your loot list into your inventory towards a relic.
In my opinion, "It's my drop and I can do what I want with it." isn't a valid excuse, because the only reason to lot on a seal is to use it for the Relic quest and only the Relic quest.
Selling a seal is even worse, because the person doing this is basically turning a profit on an item earned by 7 other people and hindering someone else' progress on Relic at the same time--and if someone I'm in a party with ever does that, I'll want my cut.
If it were me, I wouldn't invite those dudes back again, and I wouldn't feel bad to kick people and break up a party because someone doesn't want to play it fair.
Edit:
One other thing worth mentioning is that people should consider whether they even want to lot on seals, depending on how serious they are about getting relic. Will you defeat Garuda and Ifrit Extreme before 2.0 is something to consider.
Personally, I've chosen not to work on Relic until 2.0 is released because the quest should be different by then and I am willing to wait and see if it is something I want to do at that point. Right now? No. So when I do Hamlet, I don't lot on seals (just gear), and I would be pissed if someone present either exploited dice rolls like this or sold a seal and profited from the generosity of my help.






