Although I can figure out what you mean, neither of your options sound like terms I would use.
If both companies are selling genuine products, then Shop B is doing nothing wrong, they are simply
competing with Shop A. They noticed, through research, that a particular product is making money and started selling themselves. This is sometimes called
jumping on the bandwagon, especially when lot of people start doing the same thing. This is especially true of "fad" products.
If Shop B has copied the design1 of Shop A's product and is manufacturing it themselves, usually to sell at a lower price, they are
counterfeiting that product.
If Shop B is selling a genuine product, but at a lower price, they are
undercutting Shop A.
If Shop B is selling a genuine product, made by Supplier 1, but they bought it from Supplier 2, then they are
reselling the product.
1. Or recipe, or algorithm, or any other intellectual property.