Hi Ramona and welcome to the forum,
In regards to the first part of you post, you seem to have combined the topic of due diligence and marketing together. Doing your due diligence in e-commerce terms is essentially doing an assessment of both a products market potential and that of the supplier you are wanting to deal with.
That's a fundamental step of e-commerce. So first off you have to identify the market for these products, then see if you can strike a deal with these suppliers that will allow you to position yourself within that market in a competitive way.
So doing your diligence is separate to marketing hooks like bumper stickers and such. Personally, bumper stickers would be at the bottom of my marketing strategy for a product like these. I would personally be looking at developing my own website, using sites like eBay as a marketing funnel, not a sales channel and then employing a strong Social Media marketing campaign to build brand recognition.
I would see that as a much better long term strategy.
I think this starts with you doing due diligence on your market to first find out the viability. Then you will know the prices you need to negotiate to get into these markets in a competitive way and then that puts you in a much better place to then speak with the suppliers.