At worst, it would be a violation of the EULA. That is not a legally binding contract. At most, if you violate your end of the agreement, then the manufacturer does not have to uphold their end (which is usually the warranty and perhaps access to online services).
Technically, that same agreement says you can't sell the computer unless you provide all of the original install discs that came with it. It's also the agreement that says you can't jailbreak your phone. But people do, and the courts (so far) have said that is legal.
The goal here is to prevent software piracy. In the old days it was easy: just provide the install disk when you sold the computer.
I think you have 6 options (lets assume you save your old drive):
1) Sell it without a hard drive. Tell your buyer he is on his own to get a hard drive and the OS.
2) Sell it with a blank hard drive. Tell your buyer he is on his own to get the OS.
3) Install a hard drive, install whatever came with the Mac. Tell the buyer he is on his own to get Lion.
4) Install a hard drive. Buy the Lion USB installer for $69 here:
http://store.apple.com/us/product/MD256. Nobody violates anything.
5) Install a hard drive. Install Snow Leopard (or whatever) and hand the guy $29 so he can get Lion from the App Store. Nobody violates anything.
6) Throw caution to the wind. Install a hard drive, install Lion from your account. One or both of you are violating the EULA (You sold it with Lion, he has an OS he didn't pay for).
Pick the one that you are comfortable with. I'd probably go with (3) or (5).