Hi! It seems like you're early on in regards to developing your idea still. (Not saying you haven't thought of it for a while) It appears that the idea might still need more refining. The hardest and most valuable things an entrepreneur can do early on is simplify the business and idea as much as possible. You might feel like your giving up a apiece of the pie that can also be tapped into but you need to focus in one value only. That will help the market entry a lot by having a simpler pitch, marketing, value proposition, and finances.

Now.. To your first general question (assuming the core idea is financing) simplify a message and value proposition, conduct simple surveys on both bike owners & banks to really dwell into the financial reasoning of why they don't and also a glimpse of the market share by knowing how many on average would opt for financing a preowned bike vs. buying it cash. With that you could start pitching potential investors who would provide you with your website/marketing, financier insurance, legal fees & obviously the reserves and premiums to finance the bikes... An investor once obtained will have his/her own demands for equity or interest/royalties and that will help you calculate your average interest on loans offered...
I really recommend you stick to one valuable offering then as you establish the business and grow your stake in the market that you offer more services like handling the selling and buying,etc.. From what you listed it looks like your bp covers several possible propositions.