If the seller or prospect doesn't respond to you, it's their fault, give some reasons:
He has received a lot of replies and is taking a while to process them.
-> Please wait until he answers you.
He doesn't have regular access to the internet, so he hasn't read your message yet.
-> Imagine you are working or on holiday. Do you check your phone all the time or do you have it possibility?
A message may have arrived in his spam mailbox.
-> We recommend that you send a message again in this case. These can also always be found in his message box. If necessary, you can also call him on the telephone number given in the advertisement.
He's not interested in your offer.
-> If buying or selling is difficult at first, do you really still want to buy/sell the item to or from this person?