If OpenBazaar has listings including drugs / weapons etc on there then I won't be using the platform and I know of many other legit retailers who would follow the same principle.
Its going to be further complicated by individual country laws ie. UK You can sell either seeds or hydroponic equipment but you can't sell seeds and hydroponic equipment together because we have laws covering incitement. It is illegal to germinate a cannabis seed in the UK so listing a propagation light or advising a customer on growing techniques along with your seeds is covered by encouragement laws. It is not even necessary for there to be evidence of a crime being committed to secure a conviction for the incitement part.
If the items you list are legal in your home country and in the country you ship to then I see no problem. It could get messy very quickly though.
In this case you could write an app called "UK Seed Bank" and include every single seed, including marijuana seeds only in the listings, since OpenBazaar is decentralized and your app just needs to query the OB API for seeds and nothing else. You could additionally monetize your app by allowing OpenBazaar vendors to advertise on it, so when you open the app there's a rotating "Habeeb's House of Khat Seeds" banner on the top or manipulate the listing so your paid advertisers are listed first.