A scenario.
Someone asks for a ticket from another station. You decline to sell it. That person complains.
Will the senior person you had a row with and who told you not to sell that ticket stand four square with you and back you up, taking the blame on themsleves and saving your skin?
mmmm, thought not.
You only have one backside, it's your job to cover it.
One question is, what would the 'penalty' be to you, as the individual found to have done wrong? Words of advice from management or dismissal? The answer to that depends on whether it's worth creating an issue that may have implications, such as future bad relations with your colleagues / managers / employer.
Having an audit trail of emails on the issue helps you protect yourself. Relying on recollections of verbal only instructions isn't much use for any future 'defence'.
1) as advised up thread, write it all down, dispassionately, send it up the management chain.
2) ask an innocent question (in writing), such as "I'm not sure whether I should sell these tickets or not, could somone please advise in writing?"
If you are in a union, speak to them first. If you're not in a union, join tomorrow then speak to them. Union reps I've met tend to love this sort of thing.