just bought a used macbook air

Since it has no HDD and installation DVD in the first place, but an SSD and a recovery partition, you can try this:

OS X Lion: About Lion Recovery
Lion Recovery Disk Assistant


Where did you buy it and under what conditions was it sold? Maybe have a link?

What do you mean by "no hard drive"? The MacBook Air uses an SSD; do you mean the original owner took out the SSD drive? If so, you will have to buy a new one and install it yourself.

Lion does not come with installation CDs/flash drives, instead there is an online service that you can use to download the operating system.

spookyg said:

my dad bought it from a computer store that sells used computers and parts. So i'm planning to buy a hard drive, can i purchase the mac osx lion and just install it on the new hard drive?

Click to expand...

Okay, but know, that MBAs don't use standard SSDs, but custom made SSDs, which can be bought from OWC/MacSales here, and you could buy this to use as installer medium.

You could also buy a stock SSD on eBay that's been pulled out from an MBA. 256GBs typically go for around $300ish for buy-it-now (from what I've seen at least).

And instead of buying a Lion USB, you could always make your own for a lot cheaper.
http://lifehacker.com/5823096/how-to-burn-your-own-lion-install-dvd-or-flash-drive

Most important question here should be: Why did you buy this? Was it because it was an extremly attractive price?

You should not buy any used equipment unless you can verify that thing works.