Item demand =D

[Neuneck]

Those poison spores south of payon (also to be reached from Alberta by West West South) also drop Karvo with a 1% chance, so there you might get a decent ammount of zeny for your efforts, beware however of the dragontails that are on that map.
Thara Frog cards are always in demand, and you'll get at least 5 Mil from it, that would be selling it below value already. The cave east of Comodo holds them, alongside with some hydras. There is no really dangerous monster there, and the frogs themselves should die fast!
Peco Peco Egg cards are fine, too, if you don't mind the boredom of hunting their card. They drop empty bottles as well, i think. You find a lot of those east of Morroc. I think you'll score somewhere about 6-7 Mil with a card here.
If you don't want to loose out on leveling, from the 70s until the late 80s Byalan 4 offers a place that is decently mobbed and no card you'll find there is valued less than 3-4Mil, Marc can go up to like 8 Mil even! A wind weapon will help here, also don't forget to bring flywings for the 3 mermen here, if you're still lower lvl.