Board Game Arena

I'm using a black market for the first time. The system prompted me on one occasion,but didn't prompt me on two other two times when I had enough doubloons to pay for the building I wanted without using it.But I might still have preferred to use it and save one or two doubloons.Shouldn't I be able to - and if so,how ?

If you have the coins to pay, you can't use the black market

Thanks,Otreblabla.The explanation on the black market doesn't say that,it's hidden away in the italics of the expansion rules.Even then,it's not clear from the line 'you may only use the black market to build',but only clarified by 'you may not have any doubloons left after building'.