This is what I did:
0 - 98 Atrophic Sap
99 - 172 Calcium Rot
173 - 258 Cyclic Vertigo
258 - 275 Spirit Of Sloth
276 - 300 Soul Burn
The first four are made from all storebought components so it is just buying a ton of reagents and then spamming the combine button - this should take you a max of a few hours to burn through. The last one has a single farmed reagent which drops very frequently in Overthere. Soul burn is also the poison that we've customized to be stackable and provides a 400 point DD.
I would suggest not selling poisons back, but using them instead to further your apply poison skill. I can't remember if you can apply poison and then do more combines in between applications, but I think so.
I'd also like to direct you to my
Poison Idea Thread for any ideas you may have regarding poisons that could be implemented.