I'll bet this is someone looking to make a fast buck and leave you hanging with nothing to install.
Each version of CS5 comes with a unique product code, and to activate that particular product code, you need unique serial number(s) for it to install. That product code and those serial numbers are registered to the users, and believe me, it's pretty extensive what information you have to provide to Adobe in order to get those serial numbers and then to install and activate the software.
Yeah, he'll give you the serial numbers after you've forked over your cash, but then when you try to install it, good luck getting anything to launch. Adobe has gone to great lengths to deter this kind of piracy, and for those of us that buy the legitimate suite, it's a real hassle. Sure there are still people who offer pirated versions on the torrent sites and give you codes to get it to run, but this jacks up the price in the end for all of us.
Buyer beware. If you want something, pay for the real thing.