When people sell something and they have an asking price, it is based on first come, first served. First come, does not mean "I want it and I'll get you the money whenever," it means "I want it and heres the money."
You want him to just take your word that you will have the money in a week? He (I assume) does not know you, so how does he know that he can trust you. What if you say you'll have the money in a week, and the next day, someone shows up at his front door with cash in hand looking to buy his stuff. Should he say "No, sorry dude, I have a buyer that'll have the money in a week." So that dude leaves and goes to buy the stuff from someone else since he has the cash.
Now you are driving home from work and you are rolling u behind someone at a light, and you sneeze and accidently your foot slips off the brake pedal and you hit the erson in front of you. You have a tiny scratch on your bumer, but somehow you scratched up the ersons bumer real bad.
The person gets out and says "Give me $360 and we won't call insurance." You figure, hey that's a lot less than what it would cost me if my insurance remiums went up. So you write the guy a check and ask him not to cash it for a week. He agrees and you leave.
Now you won't have the cash in a week anymore and Wash can't go back to the guy that had the cash in hand because he already bought what he wanted.
I know thisis farfetched, but its just how it is dude.