For me, it's about knowing where you can find the seller, either physically or virtually. Is he out there talking about his products? Is the seller an enthusiast in these things or is he selling these...and whatever else came out of the container? Has he actually built and ridden from his own product? I have heard too many people telling stories of not being able to contact the seller, not getting what they order, not getting any support. No thanks.
Are there sellers who are not sponsors here that would fit my requirements? Very likely yes....but how do you find them? How do you know? Here, you can get that information and background...and if the seller is active here, then, more than likely, they will be a sponsor.
So, does not being a sponsor at MBc make a seller untrustworty? Of course not, but it is tougher to check their background. If you have other means (besides eBay feedback given, in 99% of the time before the engine has even run) of checking the seller out- go for it. It will probably work out. Conversely, does being an MBc sponsor make a retailer 100% great, knowledgeable and honest? Nope, but if they are not...someone here will make it clear to the rest of us and the facts will be easy to find.
As for me, $10, $20, or even $50 or $80 is not worth the risk of dealing with an unknown. My time is worth much more and dealing with problems will easily eat up what you initially think are savings.