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An advertisement is almost never a legal offer, it is usually just an invitation for

you to make one. To actually have a binding contract, you need consideration, which means a real

exchange of value. A promise made out of gratitude or for a past favor doesn't count.

You can legally revoke an offer any time before it's accepted, but be careful with

the mailbox rule. The second an acceptance hits the mailbox, the deal is live. And if you change

even one term when accepting, you haven't agreed to anything; you just made a counteroffer.