Sounds like you have identified some products with high demand, which means you will likely have significant competition as well.
To wage a competitive marketing campaign you will need a reliable supply source at an equal, or better, cost than most of your competitors. Your best bet is to identify and go directly to the manufacturer. If they deal exclusively with distributors they will let you know and refer you to their master distributor.
Once you have your supplier lined up and your website ready to go, you can launch your marketing plan. In addition to SEO, you should look at all common marketing and promotional channels. PPC advertising is usually a good place to start, though be sure that you have experience in this channel or hire someone that does.
Your PPC campaigns will help you gather essential data to inform your SEO campaigns, allowing you to focus your efforts on where it will pay off the most. Also, the rapid feedback and the ability to run controlled tests in PPC campaigns will help you optimize your landing page testing and tuning for conversions, which is essential to excelling in a competitive niche.