Very rarely does this happen. Sells are all about reputation and repeat customers. You only do yourself a disservice by stealing money since your name will get passed around and near universally black-listed. And considering most sell groups are apart of large FCs, the last thing they want is the reputation of associating with someone who steals since that will make potential buyers wary. Basically, it's not worth being an asshole.
You do realize you're comparing a free tool which displays damage numbers and calls out mechanics with buying content clears for hundreds of dollars; over a thousand if it's Ultimate, yes? Making this comparison really does show you have very little idea of how the raid scene works. The amount of people willing to spend the money asked will always be staggeringly small. If sells were frequent, you wouldn't see groups slash their prices from $1,200 to less than $600 in a matter of weeks. In fact, as Penthea said, you wouldn't see $1,200 prices period. Once a market becomes saturated, its value sharply declines. Look no further than the in-game Market Board itself. All that crafted battle gear used to cost a fortune for several weeks since crafting it took absurdly high stats and dedication. Those prices have long since plummeted due to crafting being significantly easier in Stormblood.