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.