The chances are very strong that you'd simply butcher the pig yourself - every step of the pig's life, from piglet to sausage, would have been with the same people. Pigs would have generally been killed in late autumn - as indeed would most large animals - with only breeding stock kept over the winter months, to cut down on the amount of winter fodder needed. It is possible that this was a communal activity - that pigs (which would often have been running half-wild through the summer and earlier autumn) were rounded up, slaughtered and butchered by a whole village - but there still would not have been professional butchers working there.

In archaeological terms, this can be seen in many places from the ways in which remnant bones are cut - they're clearly amateur work, as compared to the expert work done by butchers on larger estates or in monasteries. Even those, however, wouldn't have engaged in commercial transactions as such.

Sources:

Audoin-Rouzeau, F. 1987. Medieval and Early Modern Butchery: Evidence from the Monastery of La Charite-sur-Loire (Nievre). Food and Foodways 2: 31-48.