Many local game stores will take non-foil commons and uncommons for about $2 to $5 per 1,000 cards
How much is a sealed OP-03 booster box
A friend of mine, Wes, has a Zurgo Helmsmasher Mardu Goblins deck that is slick, and Zurgo Stormrender takes this idea and makes it so much better
Checkout the types of surface issues you should look out for in the tabbed panel below
While just enchanting things for value is a fine, fun way to play Estrid, the Masked, things can get out of control quickly if you start playing cards like Overgrowth and Fertile Ground so that Estrid's +2 acts like a massive ramp spell, and then The Chain Veil (perhaps enchanted with a Mask aura thanks to Estrid's 1) give you an easy way to activate your Estrid, the Masked an infinite number of times, potentially making infinite mana by untapping your lands and eventually winning with her Replenish ultimate
There are hundreds of scenarios I didn't cover