Elements of a valid contract
A legally valid contract needs: an offer (a clear proposal), acceptance (unqualified agreement to that exact offer), consideration (something of value exchanged by each party), capacity (both parties must be legally able to contract, e.g. of sound mind and legal age), and legality (the contract's purpose must not be illegal).
Example 1: A trader offers to sell 50 bags of rice at a stated price, and the buyer agrees to the exact terms and pays a deposit. Identify the offer, acceptance, and consideration in this scenario.
The offer is the trader's proposal to sell the rice at the stated price. The acceptance is the buyer agreeing to those exact terms. The consideration is the deposit paid by the buyer (and the promise of the rice from the trader) -- each party is giving something of value in exchange for what the other offers.
Agency
An agent is authorized to act on behalf of another person or business (the principal) in dealings with third parties -- for example, negotiating deals, signing contracts, or making purchases on the principal's behalf. Common types include a general agent (authorized to act across a range of matters) and a special agent (authorized for one specific task or transaction only). The agent owes the principal duties of good faith, obedience to lawful instructions, and reasonable care in carrying out the agency.