So declaring bitcoins illegal will end bitcoins as we know it today.
How do you declare bitcoins illegal ?
As long as I can send a signed (transaction) message or a private key to somenone, bitcoins can be traded, right ?
A business accepting bitcoins can display prices in fiat as long as bitcoins are traded and they can make a reasonable assumption about a conversion rate.
The shoppers are not obliged to pay in bitcoins anyway.