germany
Yes, such clauses are common and useful. § 437 BGB describes the rights of the buyer in case of defects, such as reducing the purchase price, demanding cure, or revoking the contract. But these options only exist “unless otherwise specified”. That means an individual contract can exclude these rights (commonly called a Gewährleistungsausschluss).
There are, of course, some limitations to the right to disclaim warranties.
- The disclaimers are ineffective if the seller keeps a known defect secret.
- The disclaimers cannot absolve the seller from all liability.
- While disclaimers can be part of individual contracts, they cannot be part of pre-formulated general terms and conditions (Allgemeine Geschäftsbedingungen, AGB) since they would disproportionally disadvantage one party. The relevant limits are given in § 309 BGB.
Thus, such warranty disclaimers are common in private sales, but not generally possible in a B2C context. There is however some debate how a warranty disclaimer should be phrased properly. Your phrasing “private sale, no warranties or returns” might not be effective e.g. if its pre-formulated nature gives its the character of AGB, or if the unqualified disclaimer of warranties is construed as an invalid attempt to disclaim liabilities that cannot be disclaimed.
It might also be worth discussing what a “defect” is in this context. The sold goods have qualities that the parties agreed to, or that follow from the general purpose of the goods. If the goods do not feature these qualities, the goods are defective and (by default) the buyer can ask the seller to fix this, for example by withdrawing from the contract. In retail, it is also common to accept returns of non-defective items. But such returns are entirely a matter of goodwill and do not have to be disclaimed.