Yeah, sure, Mindslicer was a one-of card in the sideboard of a Birthing Pod deck in Modern back when the Pod was legal in the format. And in Legacy, people were brewing with Mindslicer back in 2009 along with cards like Diabolic Intent and Howltooth Hollow.
Still, whatever tempted some Magic card speculators to buy it out, Mindslicer is actually a decent card… That is, Mindslicer is pretty good in EDH.
Mindslicer’s effect is quite straightforward.
When Mindslicer hits the graveyard from the field, each player discards his or her hand. Discarding your hand doesn’t sound all so wonderful. Of course, if your hand is already empty (known in Magic as being “hellbent”) the effect of Mindslicer can't hurt you.
In EDH, forcing your opponents to discard, as well, can be quite beneficial depending on who your Commander is. The Commander most traditionally tied to this guy is Chainer, Dementia Master. Chainer is all about reanimating creatures from graveyards and timed correctly. With Mindslicer, you can empty everyone’s hands and bring back all of the best creatures that they had and proceed to beat down before your opponents can rebound. It’s easy enough to simply sacrifice him in just about every mono-black EDH deck out there, so the timing of his death is usually under your control.
Malfegor is another Commander who has benefited from Mindslicer’s ability. His own ability forces you to discard your hand. Each of your opponents then sacrifices a creature for each card discarded this way. Decks built around him tend to be discard-based decks, so while not a perfect fit, Mindslicer does work his way into the gameplan occasionally. Another deck that loves discard is the casually popular Nath of the Gilt Leaf, who benefits you by giving you a 1/1 green and black Elf token from each card an opponent discards. In both of these decks, Enchantments such as Waste Not, Liliana’s Caress, and Megrim help you benefit from those discarded cards. In essence, Mindslicer is just a mass-discard spell.
Reanimator decks such as those led by Karador, Ghost Chieftain and Meren of Clan Nel Toth have tried out Mindslicer, as well. A bunch of other decks use it as tech but it hasn't become a staple in any of them. So Mindslicer is a decent card, but there isn’t an obvious major demand.
Could Mindslicer ever be good in Modern?
As a 4 mana (2BB) for 4/3, Mindslicer has a decent aggressive body. In a deck like 8-Rack, it could be a good way to force a whole bunch of discard triggers. It makes the “Rack” Enchantments Shrieking Affliction and the original “The Rack” that much better. You could replace the 4 copies of Liliana of the Veil with Mindslicer for a budget version of the deck. Perhaps that idea is what created grounds for the buyout.
A version of Modern 8-Rack with Mindslicer would probably also main-deck the enchantment Waste Not, to make even better use of the discard triggers. But is discarding your own hand going to be worth the payoff of playing Mindslicer? That’s something that would have to be tested, but it doesn’t seem to be worth the obvious drawback.
Should Mindslicer ever have been over $1? Probably not. Still, when the card gained such infamy among the Magic Finance community, Mindslicer got a new look from a new generation of Magic deck brewers. At the very least, Mindslicer is a useful Commander card that became a whole lot more expensive at one point for really no good reason.