Here is a list of items that are easy to farm even for lower leveled players that will give you a boost in zenys. Not all of us can farm MVPs and high leveled mobs.
Dead Branch – Sells for 60,000z on average and drops from Elder Willow.
Maneater Blossom – Sells for 2,600z on average and drops at almost 100% from most plant monsters like Geographers & Floras. If you have a rogue, you can steal and get doubles.
Stem – Sells for 2,000z on average and are extremely easy to find even at level 30, you can rack up a lot of these. They drop in the map one map north of Geffen. If you have a rogue, you can get double per kill!
Creamy Card – Sells for 8,000,000z on average. WOW! Creamy is easy to kill and they are plentiful!
Archer Skeleton Card – Sells for 20,000,000. A whopping 20 mil. They are also fairly easy to kill and also drops other bows and Apple of Archer.
Smokie Card – Sells for 8,000,000 and are easy to kill.
Phen Card – Sells for 20,000,000 and are easy to kill.
Invest in a Mimic card and get a chance to get Old Blue Boxes that sell for 70,000z on average.
You can also get a Myst Case card for additional gift boxes that can sell or be opened for items to be sold.
A Witch Pumpkin Hat can get expensive but is a good investment in making additional zeny. It gives you a chance of getting pumpkin pies which can be sold.
There are a lot of other items that sell for a lot and are fairly easy to get. Go on ragial and do some research!