Cutting the internet off may be promlematic if it is her only entertainment/socialisation/interaction.
With anxiety issues It's not so likely that it will cause her to just leave home and do picture selling/camming elsewhere, but it could. She's already surprised you and could do again. She may have just found her way of making money and feeling better about herself, (however you and others feel about it) and then if It's gone, it may not have the desired effect on her nor her relationship with you and the outside world.
Couple of points regarding photos sold on most of the sites of that nature, people who buy them do not 'get them'.
They get access to them. They cannot right click and copy, they just pay to view them for a while. I'm sure there are means around it for the computer savvy,but as a rule.
And paypal do not accept 'adult' transactions. If she is actually selling pictures using paypal paypal are very good at finding out and they will put a stop to her account. Also she could be reported if anybody finds out (hint?) :)
Anyway, personally I don't think a legal adult should be subject to somebody (parent or not!) going through their private email account or data online, but as you have-I would subtly 'findout' and if you're not comforable with her doing this under your roof then of course you're entitled to insist that she doesn't continue it .
I wouldn't admit I'd been through her private things but then I feel strongly about things like that, others may not-just one opinion.