Answer
A contract of sale under the Civil Code (Article 1458) is a consensual contract — it is perfected the moment there is a meeting of the minds on the object and the price, and no delivery or payment is needed for it to exist (Article 1475). Its three essential requisites are: (1) consent, or the meeting of the minds of the seller and the buyer; (2) a determinate (or at least determinable) subject matter; and (3) a price certain in money or its equivalent.
Once perfected, the seller is obliged to deliver and transfer ownership of the thing sold and to warrant it against eviction and against hidden defects, while the buyer is obliged to accept delivery and pay the price. Ownership generally passes upon delivery — which may be actual or constructive, such as the execution of a public instrument — unless the parties stipulate otherwise.