Not sure if this is entirely relevant, but here goes anyway.
Though I never buy GTCs, I do sometimes get money for entirely OOC reasons (I once wrote something in the EVE Livejournal community that was apparently so funny it was worth 100m ISK to one of the readers). If that happens, I have a way of explaining it IC, which is as follows:
Andreus' father is the current patriarch of the Ixiris family, which all told owns roughly 18% of Calluya, Andreus' city of birth, and a raft of other business interests across Intaki. Andreus despises his parents because he feels that they were cold, distant and cared only about having a son that would be the next family patriarch. They showed him little love or attention when he was growing up, and bustled him off to a fast-track program in the Centre for Advanced Studies when he was only twelve years old. He hasn't been to Calluya for more than six hours at once since - to everyone else, he pretends like it's the absolute best city in the universe, but secretely he despises it because it reminds him of his parents. When he decided he wanted to be a capsuleer, his family cut off his scholarship, wrote him out of the will and disowned him - he got a sponsorship from the Federation to continue his education instead.
If you've ever wondered why Andreus is unwilling to talk about his past or his family, this is why.
Ever since, he's been sinking a little bit of ISK here and there (remember, even a few thousand ISK goes a very long way on planets) in his own little private war against his own family. He's slowly buying up controlling interests in various planetary corporations throughout Calluya, and with the new treaty allowing capsuleers to establish their own planetary facilities, what he can't buy up, he's been directly outperforming with his own subsidiaries. His long-term goal is to put his father out of business and force the old man to acknowledge the life he's made for himself.
Any time Andreus gets money for OOC reasons, I just put it down to "some of his investments returned significant profits" or "he's been plundering one of his new acquisitions".