Yeah. It's bad. I'm not buying at asking price right now.
The problem is if this continues there will be a lack of popular cards in circulation in the market and in people's decks. If these cards are not returned into circulation (or people start selling out of their stock), prices will remain high due to decrease in supply.
Ubi can always do a full rollback of their market DB. It wouldn't be such a big issue,only disturbing players who used dirty credits to build their decks (even if they didn't know it).
It would be a huge issue, because it disturbs almost any player if the transactions from more than a week are gone. Furthermore you should also consider that some people (like me) are still buying packs. What should happen to those? Should they have to gamble again, just because Ubi didn't manage to close the account of a hacker in time?
I would probably immediately quit, if they make a full rollback.