Annie wanted to sell her collection of books and hence, placed the following online advertisement:

"Are you interested in expanding your book collection? Novels, textbooks and non-fiction books are available for sale at $10 each. The entire collection is available at a special price of $200! First one to call me at 9123-4567 will get a bonus discount. Call me for more details."

Bob saw the advertisement and was the first to call Annie. Over the phone, he told Annie that he wanted to buy the entire collection and could meet her that very day to pass her the money. However, the call quality was very poor and Annie could only discern that he wanted to meet. Nonetheless, she agreed to meet and gave details for the meeting.

Before Annie could meet Bob, she received a call from Charles who stated that he wanted to meet her and buy all the books available for sale. She agreed to meet with him, and gave him the same details for the date, time and venue as those which she has given to Bob.

As Annie was in a rush, she decided to ask David to meet Bob and Charles on her behalf. She told David that he should meet with Bob and Charles and "just negotiate and sell to whoever is willing to pay the higher price".

At the meeting, David told Bob and Charles that the books will be sold to whoever offered the higher price. Charles immediately offered to pay $400 for the entire collection. David agreed but Bob was very upset and claimed that David has no right to change the terms of the offer in the online advertisement.

Explain and discuss whether the agreement entered into by David is valid and binding on Annie. In your analysis, analyse whether David is empowered to act on Annie's behalf at the meeting and whether the four (4) elements of a valid contract are present.

Support your analysis with reasons and relevant case law.