I don't know much about Portugal, but every country I seen so far have something called sole proprietor type of business: it is usually single guy doing some work and filling taxes by himself.

Too much trouble - go Bitcoin.

I don't know about Portugal, but in the UK one of the big reasons to start a company is to get limited liability. If someone successfully sues your company, that's bad - but if you don't have a company then they'll sue you and take all your possessions, like your house - much much worse. Bitcoin does absolutely nothing to avoid that scenario. (IANAL etc).

I am wondering about Spain, but I assume the liability aspect is the same in all european countries?

no each European country has its own laws and even EU directives can vary widely as to how they are applied compare TUPE in the UK and Spain

What makes you think that?

Using bitcoin will only have the effect of restricting the OP's potential number of customers, with no effect whatsoever on the OP's liability if the laws of the country are violated.