Ali is an antique dealer and one Saturday in Nov 2007 he put a vase in the window of his shop with a sign which stated 'exceptional piece of 19th century pottery - on offer for 500. Ben happened to notice the vase as he walked past the shop and thought he would like to have it. Unfortunately, as he was late for an important meeting, he could not go into the shop to buy it but as soon as meeting ends he wrote to Ali agreeing to buy it for 500. letter was posted on 11.30 am.

on the same day later, Chet visited shop and say he would like to buy the vase at 400 but Ali refuses and would accept it only for 450, but Chet refuses and left. on his journey home Chet realizes that 450 is a very good price and immediately wrote a letter to Ali to buy it. his letter was posted at 12.30pm.

just before closing time at 5pm Di come into the shop and offered to buy the vase for 400 and Ali agrees to sell it and Di promised to return on following Monday with money.

On Monday Ali received both Ben and Chet letter before Di could come and collect the vase and pay.

Required: from point of view of the law of contract advise Ali as to his legal relations with Ben, Chet and Di.