Does a court-approved sale protect me from the seller's old debts?

It can, but only if the order actually says so, and only to the extent it says so. The specific protection buyers seek is a vesting order — a court order approving the sale and vesting the purchased assets in the buyer free and clear of the specific prior claims, liens, and encumbrances the order identifies. That's what gives a court-approved sale its real value over simply buying "as-is" from a receiver or trustee without one.

Not every court-approved sale automatically includes a full vesting order, and a vesting order doesn't necessarily clear every conceivable claim that might exist — its wording controls, so a buyer needs to look closely at exactly what the order covers rather than assuming it clears everything. It also doesn't protect you from liabilities you separately, expressly agree to assume as part of the purchase. If protection from the seller's old debts matters to you, make obtaining a properly worded vesting order a condition of closing, and have it reviewed carefully before you rely on it.