Lately, I've been seeing a lot of shops selling large stacks of items (20+) for much more sP than they used to be. The people seem to buy up every item on the shop search and then adjust the price for whatever they want.

It just happened to an item purchased from my shop. The person is selling nearly 40 of this item for 9 times what it was previously priced.

I've noticed this as well. :/

There isn't any rule against buying up a bunch of an item on the shop search and pricing it in your shop for a higher price. Although I can see it being against the rules if someone does this and then tries to use the inflated price to scam others.

I sold an item for 100k. This person now has almost 40 of this item and owns all but one of them left on the shop search priced for just under 1 mil.

It still isn't against the rules. Now if you were to go bump up the price of some normally cheap item all the way up to 45M all by yourself and then offer that item on peoples' items worth about 10M just to trick them into thinking they are getting a great deal, then that is where the line can probably be drawn. While the item may be priced for 1M when it used to be 100K, the question is: do you think anyone would actually buy them for that price?