Hi Jens, first of all welcome to the wonderful world of SEO were nothing is 100% sure but everything is changing and worth testing, you'll be learning by making mistakes, if you don't make mistakes you're not doing anything. Many times is better to make mistakes because the road to the right path is to know where you shouldn't go avoiding bad practices.
Having said this I think you should start on organizing your inventory. Start by creating categories around your best products. Try to create new categories around trafficked keyphrases and try to achieve traffic to your listings. After having organized it create a really usable website with filters and microdata which will help users find reviews on your products making for them easy to choose which one is the best and in whatever moment be able to buy it in the shortest funnel possible, every click they need to do will make them think about giving up the buying cycle.
On the prodcut pages try to build easy and clear title the same product name wills tart delivering traffic by itsefl if it's clear what you're selling.
Also try to use a secure payment to make users feel safe when buying on your site.
Nowadays it helps a lot morehaving a really good onsite seo than build a lot of links to improve your visibility to achieve traffic. After you build a good site, improve your contents, descriptions, photos, reviews, filters, and you'll start achieving some links, you may start consider building some quality links and social media mentions as you may have read in the seo guide seomoz provides. TRy to avoid paid links and use only highly themed pages related to your website.
Adwords may also help you delivering traffic faster but try to optimize a lot your spent to make your first sales and help your site survive until your seo improves!
Hope this may help you starting!