Here's how you buy them. Go to his place and have this conversation:
"Hi. How much you asking for the letters? I really, really, want them for my garage and I can't find them anywhere else. You've got like the only set there is!"
"Well, I'm asking $200 a letter, so that'll be $1,000 for the set."
"Wow, that's a bit more than I was expecting. I only brought $400 with me, but you're asking a fair price. Okay, well I only have room in the car to take two of them anyway, so here's $400 and I'll run two of the letters home and stop by the bank on the way back."
You give the guy the money, and grab the O and the D, load them in your car and go home. He'll be thinking there really is a sucker born every minute as he counts his money and waits for you to come back, and he knows you will because you just had to have them. Wait an hour and then call him up and say you had a chance to think it over on the way home, and you really don't want to spend another $600, so you'll just keep what you have and we'll call it a day." He'll insist you bring the O and the D back because no one is going to buy D G E because they can't spell anything with them, and you say "I hadn't thought about that, but you're right. Tell you what, think about how much you really want for those other three letters, and give me a call back." Once he gets down to $200 for the other three, come get them.