"We are selling our small dog."
Translation:Wir verkaufen unseren kleinen Hund.
Unseren: "Hund" is masculine akkusativ here (the direct object of "verkaufen")
Kleinen: take a look at Adjektivendungen (adjective endings): you can see it by giant tables of gender, number, case...or you can see by the thumb rules based on the preceding articles or pronouns. That tells us that with "unserEN" we must use "kleinEN"
Hope that helps
And (here)[http://www.nthuleen.com/teach/grammar/adjektivendungenexpl.html] you can find a great explanation on Adjektivendungen!
Off topic... I'm doing this sentence when the scandal with my country's politician is still fresh having produced countless memes.
The guy said on television that if people have no money to pay for a gas bill, it could be a good idea for them to sell their dog of some elite breed.
Much hilarity and indignation ensued. Could he be any more clueless and detached from average people?