We are moving to the states in late June. We own our apartment and a machsan which are officially coupled together on all legal documents. A couple live in the adjacent machsan downstairs and there are other residents of the building who could possibly interested in purchasing the machsan from us separate from the apartment. Do we have any chiyuv to offer the machsan to any of these people first because of bar metzrah even though it is officially coupled with the apartment?
No, if the machsan is coupled with the apartment, it is not considered as a separate housing unit (even though some use it as one), and it is permitted to sell the machsan together with the apartment.
However, if the people in question want to buy the apartment as well as the machsan, the halachah of bar matzra will apply.
In addition, even if there would be a halachah of bar-matzra concerning the machsan, this would not obligate you to incur a financial loss, and if it is more profitable to sell them together this is also fine. However, as noted, there is no halahcha of bar matzra in this case.