How come BTC is not a legal currency?. Its a currency.

Technically you should be able to take payment in bottlecaps for any items you sell, its gonna be a bad practise, but you can.... or not?

What I would do is list the item at xx BTC, but before putting it up I would have looked at the current exchange rate for GBP vs BTC and given myself a bit of favour on the exhange and then just account the GBP value of the sale, and not the BTC value.