As said in the answer of pinckerman: It depends.
- It increases your chance of getting enough guppy items for a transformation. This happens because it might randomly select the curse pool to sell stuff, and this pool is mostly guppy items in greed mode. Or it might select the devil deal pool, which also contains guppy items.
- It might result in ridiculously good deals in the shop or in the treasure rooms.
- If you have already a gamebreaking run it'll add devil and angelroom items to the shop pool, and that's really good.
- It lowers your chance of breaking the game because it lowers your chance for seeing blank card (blank card + two of diamonds and a battery for sale is infinite money in greed mode).
- You do not know what kind of items anything will contain, so no guaranteed shop pool items in the shop. Nor guaranteed boss room items in the bottom treasure room and in the shop. So you might run into very little or even no hp-increasing things at all.
So in the end it depends on what your goals are for the run. Do you want to play it safe and be assured that you'll keep finding hp items (since the boss room pool contains a lot of them)? Don't take it. Do you already have a way to infinite money? Take it, it'll help you get even more offensive items. Do you need that final guppy item? Take it if you like a gamble.
I personally always take it because I love the randomness, but it's up to you and your current situation. On a small note: do keep in mind that breaking the game and buying half a million items makes the game crash quite a lot, especially when you actually end up in combat. I had one run where I could fire one shot in the greed combat room, and then had to wait until everything was dead or my shot had subsided from being everywhere. Firing a second shot would crash the game. But at that point one shot would clear at least three or four waves.