Can I be forced to personally guarantee my buyer's obligations just to get the assignment approved?

No landlord can force this on you against your will, but they can make it a practical precondition of consenting to the assignment — meaning that, functionally, refusing to provide it might mean the deal doesn't proceed on that location at all. Whether the landlord has this kind of leverage depends on your lease's consent clause, but many are broad enough to let the landlord set reasonable conditions on approval, and demanding continued or additional security from the departing tenant is one form that can take, however unwelcome it is to hear as a seller who's trying to exit.

This is very different from the ordinary expectation that assigning your lease and selling the business should let you walk away cleanly. Agreeing to guarantee your buyer's future obligations means your exposure doesn't end at closing — it potentially continues for as long as your buyer holds the lease.

If a landlord is pushing for this, it's a major point to negotiate rather than accept reflexively, and it should factor into your price and deal terms. A Treadstone business lawyer can push back on the scope and duration of any such guarantee.