Since the legendaries buff blog came out, I have seen a significant increase in the price of Exq Essences and Flawless Square Emeralds.

I have about 3000 essences, and 1000 emeralds. I bought them when they were both about 800g ish. Should I keep them and wait for prices to rise, or sell them now?

you should keep them for 1-2 more days imo

Any advice on commodities will always be mere speculation, but I would either sell them now or wait until the next blog post about 1.0.4 or wait until 1.0.4 actually comes out, because that's when they usually spike ridiculously until the market corrects itself. Usually peaks on weekends until ~a couple days to ~a week afterwards.

(Hint: use the price tracker)

You should wait until 1.0.4 comes out since the weapons ilvl61-62 can have high dps, many weapons will probably be crafted, and many legendaries as well, which means there will be high demands right after 1.0.4 comes out, don't wait too long after though.