Membership in the United Nations is not tied to formal recognition by any other country. As such, technically there wouldn't be any such thing as an "automatic embargo" if a member nation voluntarily left the UN.
This doesn't mean to say such a move wouldn't raise eyebrows - and in particularly egregious situations, retaliatory action - but membership in the United Nations is a voluntary thing that is in no way associated with recognition. Any trade actions would be pursued through the WTO - which is a separate organization altogether
For example, the Holy See is formally recognized by the United States, and most of the world (there are exceptions) - but it chose to only have observer status at the UN. By the same token Palestine has observer status but is not recognized by the United States - and indeed, relations actually deteriorated when it sought to gain admittance to the UN as a full member state.
Finally, the predecessor to the United Nations - the League of Nations - had one very prominent "missing" member - the United States itself. While there were some minor privileges which the US was technically not entitled to as a result of its non-membership, it obviously wasn't that big of a deal.
In short - the UN is a voluntary membership. It represents worldwide consensus on some occassions and on some matters - but as a confederation, non-membership is not an issue.