Yes, Arioch13 is right. Without a residency permit, you must leave the country after 3 months for at least 3 months. And in no case are you allowed to work without a work visa.
I´ve been living and working in Mk for almost 4 years. I exit and reenter the country at least once a month. Last year, when our company lawyers filed for the extention of my work visa + residency permit, it turned out that by some odd mistake the residency had not been issued the year before. So, without my knowledge, I had illegally resided in the country for an entire year. As in your case, when frequently crossing the border, my residency status was never of any interest to the passport controllers. But, after the ministry of internal affairs became aware of the irregularity, I was told to leave within three days. No kidding. German kicked out of banana republic. Thanks to my boss pulling some strings, I was allowed to return after only 5 weeks and not taken to court.
In the meantime, I have heard of a number of other expats who got expelled aswell. The authorities are trying to make a point lately.
My advice is, if your income stems from business in Mk, stick to their rules and apply for work visa and residency.