As high a level as you can be...I dont think hell/nm/norm or NPC makes a difference but if you had the choice why not do it in hell (reselling you can get a better price potentially)? Oh pack a Gheeds Fortune Charm as that charm reduces gambling prices as well.

If you're one of those strange people that is looking for the microscopic chance of getting a useful unique there are certain levels that have the best chance (for instance gambling for a SoJ is best when you're level 43-47, a BK is ideally gambled at a much higher level but I seriously doubt you'll find either through gambling anyways) but for the high level mods on rares you'd want to be above 90 I think and be gambling on coronets/circlets to try to net that +2skills, FCR, resists, life..etc.