Can a seller insist the whole company be sold as one unit, with no carve-outs allowed?

Yes, a seller can take exactly that position, and there are practical reasons they might. Selling the whole company to a single buyer avoids the seller being left holding a remainder that's harder to sell or run on its own, avoids the complexity of dividing shared staff, leases, and contracts between what's sold and what's kept, and can simplify the seller's own tax and wind-up planning by closing everything in one transaction.

The nuance for a buyer facing this position is that it's a negotiating stance, not a legal requirement — sellers set it because it genuinely benefits them, and a buyer's response is to weigh whether taking on the unwanted parts is worth it for what they actually want, rather than assuming the position can simply be argued away. Sometimes it can be addressed indirectly: buying everything but negotiating a lower price for the parts you don't want, or agreeing to take them on with the intention of reselling or winding them down yourself afterward.

If a seller won't budge on this, a business lawyer can help you assess whether the whole package, priced appropriately for what you don't want, still makes sense compared to walking away.