Are you doing this job part time or full time? Are you new or long time in the business?
Personally, I think you dont have the right attitude as a seller. We as buyer do not want to hear excuses or reasoning, that is not our problem. You have guaranteed of the item, and the buyer has already paid based on your agreement. If any case arises, you as the seller are held responsible. So it is up to the task of convincing the buyer that the product will arrive either on time or given a new lease of deadline for them to hold on to. At the same time, you're saving your reputation as a respectable seller. It wont help if you're being defensive about the matter, because in the end....its the buyer who has to take the risks of losing his money. Of course, you can explain of the current situation, but do not expect that after explaining that it is NOT YOUR RESPONSIBILITY for the arrival of the item, that you can just let the buyer wait for days, weeks and months.

And he is buying in bulk, which normally means he is intending to sell it off. If due to the delay of arrival, the RAM price has gone down....isnt that meaning that he is standing to lose the value of the RAM itself in the market? I hope at least you can care enough of your buyer to at least give them a confidence call and reply to their inquiries. THAT is how you maintain your reputation my dear friend.