".Can I be arrested for dealing in marijuana ?."
Stock has nothing to do with illegal cannabis in other states. Please don't be this clueless.
I would say if the stock is legally traded you wouldn't get into any trouble. There are pesticide companies that make bug poisons that are illegal to use in Indiana, but you can still trade their stock. Tobacco companies today enjoy much more freedom in Asia and Europe but you can still trade their stocks. Should be the same with marijuana.
Trump can't legalize marijuana by himself. It would require an act of Congress. Frankly, I think it's not clear yet whether Trump and the Republican Congress are going to be able to get along and work together. I'd think most Republican congressional leaders want to look 'tough on drugs' so they're very unlikely to reclassify marijuana.
President Obama decided to allow states to legalize pot if they wanted to, despite federal law. Trump may well decide not to continue this policy. Jeff Sessions (Jefferson Beauregard Sessions), Trump's pick for attorney general (not confirmed yet) is a rabid anti-drug warrior. So at some point he may decide to reverse the legality of pot in Oregon, Colorado, Alaska, Hawaii, California, etc.