A: This is one of the more interesting Magic the Gathering finance related questions I’ve been asked. It really comes down to what is in those 100 cards. If it’s mostly land cards, then the stack is relatively worthless. If it’s 100 commons and uncommons, typically no vendor will buy them, since typically stores buy in bulk of 1000 at a time and pay about $3-5/thousand.
If there are rares in the pile, you’re looking at least $0.08 to $0.10 per rare, and likely more depending on what the rare is. A stack of 100 rares, for example is going to get you likely $8 or more from a local game store that buys Magic cards. But it’s more likely to be a mixture.
It is true that you can buy lots of 100 Magic the Gathering cards for anywhere between $4 and $8 online. Some have more rares than others, and some will have more duplicates than others. But as far as selling them yourself, if there’s nothing of substantial value among the 100 cards, you’re probably looking at a value of between $0.25 and $1.
You can learn a lot more about what Magic cards are worth in my post: Magic Card Values: What Are My Magic the Gathering Cards Worth?