1. Both bid and acceptance are in past tense, e.g.: Bid to buy: "I have bought this car for USD 1000." Acceptance: "I have accepted."
2. Both bid and acceptance are in present tense (provided that the intention is to affirm and accept, not haggling,) e.g.: Bid to buy: "I buy this car for USD 1000." Acceptance: "I accept."
3. The bid or the acceptance is in one tense and the other in another tense, e.g.: Bid: "I buy this car for USD 1000." Acceptance: "I have accepted." Bid: "I have bought this car for USD 1000." Acceptance: "I accept."
Imperatives (orders) and questions are not considered valid as affirmations, such as, "Sell me this car for USD 1000!", "Do you sell me this car for USD 1000?", and "Buy this car for 1000!" These statements are part of the bargaining process, and are not contractually binding. In marriage contracts, unlike in buying and selling, orders are considered affirmations, such as if someone said "Marry me for a dowry of USD 5000!" and the other answered, "I hereby accept your marriage." This is because haggling is not normally part of the marriage process.
If one of the parties makes an affirmation (bid) to buy or sell, then the counterpart has the option to accept during the contract session, as long as the bidder does not withdraw his affirmation (bid). This is based on the hadith of Tirmithiy that "the buyer and seller have the option as long as they did not depart from each other."
The contract session lasts as long as the two parties remain in their place and they do not engage in unrelated conversation. If they are walking together, then there must be no moment of silence between affirmation (bid) and acceptance.
If affirmation (bid) is followed by acceptance in the contract session, then there is no option to withdraw the bid or acceptance, even in the same session. This is because in the Quran (5, 1) we are ordered to remain faithful to our contracts: