Peasants were the largest demographic group in Latin Christendom, comprising somewhere around ninety percent of the population or more. Most of these people were tenant farmers who leased a portion of a larger slice of land in return for a rent of service, kind, or money (although the latter is more rare and often only found in the later Middle Ages). These people could theoretically support themselves and their family off their own land. Some of these peasants were not free—serfs—meaning, depending on when and where they lived, they may be bound to the land that they worked and could not seek legal redress outside of their primary lord (the kings justice, for example, would have been beyond the reach of these people). All of this is to say that medieval peasants, even if serfs, were not slaves—a misconception that comes from the continued use in some medieval sources of the Roman word for slave, servus, and should not be taken as a indicator of shared meaning. I've seen documents that use servus, rusticus, and—in the vernacular Old French—vilain. Outside of what they needed to pay as part of their obligation, they were free to sell, trade, or work elsewhere in the hamlet or town for wage or kind.

The details for the sort of trade you are asking about is difficult to trace since the economic lives of most peasants only appear in the records of lords and local courts when they have some sort of legal problem or reach a certain level of wealth. The community in which most medieval peasants would have interacted and traded was the Parish, which—besides the family—is the basic unit for understanding peasant society. The Parish community would have operated as a sort of social nexus for the rural peasantry, through which small transactions would have been negotiated. Work/Trade for wage and work for kind probably occurred simultaneously based on need, although certain economic historians believe quite strongly that the latter wouldn't have occurred at certain points of economic crisis.

Most farming hamlets were largely self sufficient in respect to their daily needs, so the average peasant had no need to access the sort of long distance trade I think you might be imagining. This is easier to understand when you consider what we know about their eating habits. For most, diet was simple and with the exception of certain feast days fish or meat was largely a luxury most peasants could not afford. The bulk of peasant diet probably came from cereals, supplemented by whatever local herbs and vegetables that they grew in their personal garden, of which every farm was sure to have at least one. On the plus side, beer was plentiful, although it was usually not brewed at a high ABV. A quirky and active market in late medieval England was in beer, which often was brewed by women and sold ad-hoc as a means to supplement income.

I should add that the definition of peasantry is something of a thorny topic for medieval economic historians, particularly in England where they have a wealth of sources that give them a wide range of local practice to squabble about. For your purposes, I would recommend avoiding most of these debates and read Judith Bennett's A Medieval Life: Cecilia Penifader of Brigstock, c. 1295-1344, a slender volume that will give you a good general overview of medieval peasant life rich with economic detail. For a contemporary, non-economic view of French peasant life I would recommend Emmanuel Le Roy Ladurie's Montaillou: The Promised Land of Error, that chronicles life in a small southern French town through inquisitional records and provides small details about how peasants moved through the world, made friendships, and even weird things like their perception of time. A wonderful view of life peasant life a few centuries later is presented in the first few chapters of Eamon Duffy's The Voices of Morebath: Reformation and Rebellion in an English Village (2003), a book that, although about religious change, gives insight into the everyday.

edit: spelling words.