1. Adding lines into the .csv file,
2. Deleting sales from the .csv file - so the record of them buying shares is still there, but not the record of them subsequently selling them.
Haven't got a GLBSE .csv file on this computer, but last time I looked at one, I don't remember it being GPG signed or anything to allow guaranteeing its unmodified nature. It would deter casual scammers - but actually could be used to give credibility to slightly more sophisticated ones.
One thing it DOES do (in its favour) is link shares claimed to specific transactions - so anyone inserting lines would run the risk of claiming a sale to someone else was to them and having it contradicted by the genuine buyer submitting a claim on the same transaction.
My idea to prevent the addition of fraudulent buys and sell history in the file would be to compare the csv file with the GLBSE history and also against other claimants. If there are was only one purchase at a given time yet two people are claiming that they bought the shares at that time point, then both of those claims would need to be considered suspect and evaluated further.
Yeah - mentioned that in my last paragraph. The real weakness is that it allows people to delete just their sell line - and leave in their buy. With the buy being genuine (so unlikely to be contested) it would give huge credibility to someone who had bought/sold shares but was now claiming to only have ever bought them. All it can really show is that at some point they probably owned some shares - not that they still retained them when GLBSE closed down.
That is a good point that I did not think of. People would have to remove any withdraws from the csv file as well (if they made any withdraws) as they can't remove bitcoins that they aren't suppose to have.