Actually, the international version of the Galaxy S GT-i9000 uses the following bands:-
2G Network GSM 850 / 900 / 1800 / 1900
3G Network HSDPA 900 / 1900 / 2100
Chances are, the guy didn't unlock the phone but was happy to take your money. You ought to go back to the shop and make sure he has unlocked the phone for you.
For future referrence, there are a couple of free apps on Google Play that will unlock your Galaxy S for free, in seconds.
https://play.google.com/store/apps/details?id=com.helroz.galaxysunlock&feature=nav_re sult#?t=W251bGwsMSwxLDMsImNvbS5oZWxyb3ouZ2FsYXh5c3 VubG9jayJd
https://play.google.com/store/apps/details?id=com.clarkehackworth.SamsungGalaxySUnloc k&feature=related_apps#?t=W251bGwsMSwxLDEwOSwiY29t LmNsYXJrZWhhY2t3b3J0aC5TYW1zdW5nR2FsYXh5U1VubG9jay Jd