There are many things in your questions.

1. If you want to sell a similar product, it means you have identified a market/niche that is not addressed by the current product. A good thing to do would be to contact the company and check for the option to get an Agreement where you could build a similar product - based on theirs - with some tweeks, making sure it is a win-win deal for both parties (you are not competing their product and they access a new market through you). Then you would not have to reinvent the wheel, you would leverage existing IP and knowledge they currently have for this product and able to go quick on the market.

2. If first option doesn't work or you want to compete them directly, then you need to build your own product, i.e. re-invent the wheel (more costs, takes time, etc.). In this case, you need to make sure you are not infringing their IP or any other IP that other vendor might have with similar products (as correctly indicated in previous Lee's comment).

You should not view IP from a strict "law" perspective, you should view IP as an opportunity to leverage other's people work in order to quickly and efficiently build solutions that's going to solve your customer's problem.