Obviously this is a horrible idea, but I wanted to see if it would work. To do this, I took average daily price data from the client and used magic to compare prices on days of the week to prices on other days of the week for each week for each week of the year.
Plex: Over the course of the year plex have risen in price from 560 to 630 million. Going up makes it a lot easier to make a profit, but buying on saturday and selling on Tuesday yields a sum increase between those days over the course of the year of 335 million, or 6.5 million per unit (1%).
Isogen: Over the course of the year isogen has fallen from 150 to 130 isk per unit, more than negating the effect of rising prices. Buying on Thursday and selling on Saturday yields an average profit of 0.6 isk per unit (0.5%). Pointless in terms of profit, but I'm super impressed that it yielded a positive number.
Megacyte: Dropped from 2100 to 1600 over the course of the year. Buying on Friday and selling on Saturday makes an average of 14 isk per unit (0.9%).
Minmatar fuel block: Mostly steady over the course of the year. Buying on Thursday and selling on Monday yields an average profit of 122 isk per unit (1%).
Hobgoblin II: Buy on Friday, sell on Tuesday. Average profit is a massive 2700 isk per unit (0.5%).
Raven: Buy on Thursday, sell on Monday. Average profit 900k (0.6%).
Does it work?
Well, not by enough to beat the tax rate. Plus, I'm picking the buy and sell days that maximize profit over the last year, but I don't know if there's necessarily a reason they are low and high on those days or if I'm just cherrypicking the data. Even if this does work, it's just margin trading with longer durations and slightly higher margins.