Translation:Sell the house.
The ending 세요 and 십시오 have the "please" attached already. Therefore whenever you use that ending, the word please is assumed to be part of your sentence.
this isnt entirely true... -시다 expresses humility and -ㅂ시오 expresses the formal interrogitive form but often it translates in context as "please do xyz" in this case saying please would sound like a favor in which case you would add 주세요 as in 집을 팔아 주세오
Definitely. It is slightly less formal than 팔아 주세요, which is less respectful than 파십시오 and 팔아 주십시오, but all those forms are considered to have an inherent 'please'.
세요 is a polite command ending. This would translate as "Please sell the house".
No, it does not. It imparts a degree of politeness which we roughly transfer over to English as “please.”