It might be helpful if you examine the question more closely. For example, what is meant by a free market?
Again examine this more closely - what were the slaves revolting against? Being slaves - of course! Is this an economic or a political issue? In a sense, both. Spartacus and his fellow slaves wanted to overturn the prevailing economic system (slaves as property) AND the prevailing political system (slaves as free, autonomous human beings).
At the time in question (74 BCE), did Rome REALLY have a free market? It may have been free in the sense of what was practiced at the time (it was a free market for citizens of Rome; but slaves were not citizens - they were property).
However, I would argue that Rome did not have a free market as it is commonly understood today. I order for a market to be free, it requires certain things. For example, all people should be free to enter and exit the market without any undue coersion or prohibition. Free markets requires a court system that recognizes and respects property rights and enforces contracts. Free markets require some sort of standardization with respect to transactions (for example, currency and currency exchange).
One can have a market system whereby people (slaves) are seen as property. And this property can be protected via laws and the court system (buying and selling of slaves). Plus, contracts can be enforced regarding this property (I agree to buy slaves from you at a future date and you agree to sell me those slaves at that time). However, unjustly prohibiting people from entering/existing the market (if you are property then you aren’t a person) or unjustly coercing them to engage in market activity (slaves can’t disagree on whether to be sold or not; well, they can - but then you upset the dynamic of the market if you can’t protect your property or enforce contracts) would disqualify itself as a truly free market (in a modern sense).
It’s possible that a free market can lead to democracy; it just depends on how one defines free market and democracy. In other words, how many people are able to be involved in both. But it doesn’t necessarily mean that a free market will lead to democracy (or rather, if you allow greater freedoms with respect to market activity, this doesn’t necessarily mean that it will lead to greater freedom in the political arena - i.e. more democracy). Witness Singapore, which has quite a bit of freedom in the market arena, but less so in the political (democracy) arena.
Personally, I would contend that political freedom is necessary prior to the establishment of a free market. In other words, in order for a free market (as understood today) to emerge, it requires some semblence of political freedom to occur first (or in conjunction with greater economic freedom).