I don't know English too good so maybe other preposition works but Oni sprzedają dom Marcie.
Oni -they (nominative)
sprzedają- sell (present tense , plural 3rd person)
sprzedawać kogo? co? (Akk) - I sell who/what
sprzedawać komu ? czemu? (Dativ)- I sell whom? (who buys)
Dative Indicates the indirect object. To whom something is given, or action for somebody
Accusative Indicates the direct object of the sentence
(those last descriptions from https://en.wikibooks.org/wiki/Polish/Noun_cases , a place where I check the names of cases every single time )
The way I did to learn it was: the dative case kind of contains the "to" in itself. When you use the dative case, you know that's the person in the sentence getting the thing "given" (or, in this case, sold). So, in this case, when you use Marcie (which is the dative case for Marta), you know Marta is receiving the thing being sold.
I don't know if I made myself clear, but it total makes sense in my head :)
Actually I don't see anything at all in "Ona jest mamą" that would suggest she's my mom. There isn't anything about 'me' in the sentence. Compare with "Rozmawiam z mamą" (I am talking with [my/] mom) - here 'I' am present in the sentence, so it's most probable that I'm talking with my own mom. Generally this 'presumption of ownership' states that the object is probably 'owned' by the subject of the sentence, and your sentence has "She" as the subject.
As for "their house", that actually makes a lot of sense. Added.