It is necessary to consult for placing a lien - שעבוד צריך לימלך
A field that was sold שלא באחריות and a בע"ח claimed it for his debt, there is no recourse for the buyer. However if the נגזל reclaimed it, the buyer has recourse even from the משעבדי of the seller. There is no recourse in any event if the שטר specified that there is no אחריות.
Before he took possession, etc. - עד שלא החזיק בה כולי
One may cancel a purchase if he did not pay for it yet, even if he made a קנין; provided he did not have the opportunity to inspect his purchase. A debtor may pay with the (bought) item provided it retains its original price.