Tenanted from Erev Shabbos until Motzei first day chag
Tenanted again Mid Chol Hamoed until after sof chag.
Who does Bedikas Chometz? Mechiras Chometz?
If the apartment is not rented out until Friday morning, the owner of the apartment has to check it on Thursday night. If he didn’t, then the person who moves in on Friday will have to check it.
I don’t know what you mean by mechiras chometz, whose chometz are you referring to? If it is owned by the landlord only he can sell it. Any other chometz that the tenants bring in has to be sold by them.