Contract Law in Hong Kong

No, there is no contract between your friend and you.

First, we can see that your friend ask you :
"Will you sell the Ming Watch? What is the lowest cash price?"

>>This is an invitation to treat. An invitation to treat is different form an offer.
An invitation to treat means a person who making an invitation to treat is just inviting offers rather than making one.

I replied, ""Lowest cash price for Ming Watch for HKD1000"
>>This sentences also not an offer because you only supply of information that your friend request (Harvey v Facey (1983))

My friend the faxed, "I agree to buy Ming Watch for 1000 asked by you"
There was no reply to the last message.

>> 雖然Fax 係收到時已經代表張contract有效,,
但係, 我覺得有一個exception 係到...
就係朋友同朋友之間o既contract可以係唔binding,,
*** 因為有一個presumption,, 相方have no intention 去create legal relationship.
呢類只係屬於Domestic agreement.
所以, 應該冇效。
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以上只係個人觀點,, 如果有錯誤,, 不好意思 !