Will I trap moisture in the concrete of basement walls and floors if I seal them with epoxy?
Would moisture be trapped? To some extent.
Is it bad? No, once concrete has been mixed and started to cure, keeping it moist helps it get stronger and this effect can go on for a long time. In no case will it ever make the concrete weaker. See the graph at the end of this URL:

http://wgbis.ces.iisc.ernet.in/energy/water/paper/drinkingwater/wellsconstruction/appendix4.html

There are some who say the concrete in the Hoover Dam is still curing.

There is no need for concrete to breath from the point of view of the strength of the material. There will be moisture retained from the original mix and it is possible some are confused about the chemistry of concrete and think it needs to dry, but the truth is the complete opposite of that.