Would need to know the brand, and age, of the regulator. Although you can purchase a brand spanking new Taprite regulator for under $55 from Keg Connection. IMO, I'd pass on the steel CO2 tank unless it has a very recent hydro date stamp on it (early 2011 as the oldest). The rest of the kegging parts are damned close to brand new prices, or higher than new. So unless they are really, really new, or used for just one keg/batch, I'd offer far less.
Look at sites like Keg Connection for the exact same items and you'll see what I'm talking about.
Also, as already mentioned, he's not looking to sell just the kegging items you want. So unless he changes his mind, I'd pass. Or maybe offer ~$100 for the entire thing.
On a better note, Keg Connection has starter kits that include most of what you need (or will once you select the missing, and stated as such, items). You can also email/call then to get advise on what to order so that you get what you really need to do the job up right. Plus, you won't need to worry about something lingering in the keg, or regulator, that's going to ruin a batch. Peace of mind, plus not having to toss a batch [IMO] is worth the cost difference there.
