Old Sebilis is a great plat farming zone in Everquest. It is one of my definite favorite farming zones. Making plat here is so easy because you make money from plat drops (Approx 1 – 10 Plat per mob), you get vendor trash and you get stuff like defiant gear and tradeskill items that sell on the Bazaar. You can probably start earlier but to farm plat efficiently i would recommend that you are around level 70 or over. This is also a great place for people with free to play accounts to farm gold as you get big plat drops and vendor trash. The named mobs here can also have some good valuable items.
When you are ready, head to Old Seb through Trakanon’s Teeth in Kunark. The zone is gigantic and you can go wherever you want. I prefer to stay on the top level with the Frogloks as they are the easiest to kill and the loot table of the entire zone is pretty much the same. If you are high enough level you can just stack up as many mobs as you can handle and AOE them for maximum efficiency. Killing them one on one is still efficient though if you can handle big packs.
Once you are finished head back and either sell everything to a vendor or go to the Bazaar!