מוסף התנור: של בעלי בתים טהור; ושל נחתומין טמא, מפני שהוא סומך עליו את השפוד. רבי יוחנן הסנדלר אומר, מפני שהוא אופה בו כשהוא נדחק.
The additional piece of a householder's oven is clean, but that of bakers is unclean because he rests the roasting spit on it. Rabbi Yohanan Hasandlar said: because one bakes on it when pressed [for space]. See here page 27
This distinction between a householder and a professional/shopkeeper comes up in several other places as well, arising from their different uses for utensils.
If a householder sold an oven to a baker, is the "additional piece" suddenly susceptible to tumah as soon as it's acquired by the baker, or do we say that since it was insusceptible to tumah after it was completed as a utensil, it will stay insusceptible? If a baker sold an oven to a householder, does it lose its susceptibility as soon as the householder acquires it? Does it matter if it was tahor or tamei upon acquisition?