I have spent a bunch of time to try and narrow down what causes the darkened sell price, although this observation might already be known.

The glitch occurs when hovering over an icon on the minimap.
Using the regular map works fine.

I tested this with several different addons, including a fresh install while only using questie.
I have tried it with several versions of questie ranging from 2.0 to 3.1 .

Additionally it seems like Questie interacts with how normal item tooltips are displayed, here is a comparison with and without Questie with the addon PriceDB to display sell price:
http://imgur.com/S9bp5Te

Same here with v3.68. Seems to be related to #128, #146, and #225. This is definitely caused by Questie, I don't use any other addon right now.

Unfortunately, the issues were all closed without a solution. :(

I'm having the same issue with the darkened sell prices. It almost looks like the text for the sell price is behind the gray tooltip box.