If you want a zeny sink, put in an NPC every 2 months with a donation goal of 5b total or so (depending on what you think is reachable / amount may vary and adjust to current total zeny of the server) that has to be met within a week, where you can type the amount of zeny you want to donate in a text box, minimum donation amount 1,000z, and if the donation goal is reached there will be a week of double exp / tripple exp / etc. / whatever else you can come up with.
This will drain zeny from all players and will benefit all players, especially new ones.
1. will not help at all with the bot situation unless you completely remove the value of those items and have them be sold for 10z each.
Do not add those items to NPC shops please, don't!
This idea is genius, you could even set rewards in tiers depending on the amount donated. That way you could incentivize people to donate