You can only gather resources on the property that you own. If you don't have enough to buy plots of land that have gold mines on them, then you don't have a choice but to buy the gold from the person who owns the land who mined it and is selling it. In my game you can't just gather any old resource, just like in the real world you can't just go cutting down trees on land you don't own. This ownership of land to gather the resources on the land is really what forces collaboration (just like in the real world). You won't have the money, right away, to go buying up plots of land. The starting money is still up in the air but it'll be around enough to buy a 6x6 tiled plot of land. All land starts out being owned by the "government", and all money paid to the goverment will be put back into the world in interesting ways (random treasure chests in unpopulated areas for example).
If a person wants to go explore for plots that have gold on them that's fine, but if they are far away from where people start to build their towns then you'll have a hard time with transportation and getting other goods. There is no teleporting and at least to start with only walking to get around so travels are slow and risky. Even if you do buy a plot of and with gold on it, it won't have other materials you need so you'll have to buy those from others. This is what creates the dependency between players and out of this will come active markets. By the time you have enough money to go buy that plot with gold, someone else most likely would have bought it and is selling it.
Add that on to the fact that your needs simply won't allow you to be able to meet them all to the max yourself. It just won't be possible so you will need to rely on others which means buying things at a marketplace.