Keep it simple as you can. My suggestion is star to talk with your customer following the The Mom Test rules.
The Mom Test in a nutshell is:
It’s called The Mom Test because it leads to questions that even your mom can’t lie to you about.
Book some videocalls with 5-10 of your customers and ask them about how they use your product in their life or if they use it for a specific use case ask questions around the use case. For example if the use case is: saving money for my next vacations, some questions could be:
While you are talking with them you star to notice patterns and identify needs or pains, it is a good staring point to level up your product discovery to specific topics.
Product discovery is apply a lot of differents technies and tools to discover how to build the right product, but in the end all of them look to give you an undestanding of your customer, so talk with them is the basic for that.
IMO focus on the supply side first is great. To approach the sellers I would choose 1 or 2 platforms as sources of possible users, maybe FB Marketplace and Heyauto.
Then I would search for around 100 sellers on the platforms to contact them. For example, I would contact sellers by FB Messenger with the excuse of learning more about the business of selling used cars with a cold message like this: Hey Bob, i have seen you have a lot of expertise in selling used car and I want to learn more about it for my [insert any excuse here(podcast, homework, etc)]. Could we chat a few minutes about it?
You will need to reach 100 sellers to get 25 calls (a great conversion rate) and try not to sell anything, remember your goal is learn.
Learn more about the types of questions you should ask in the book, The Mom Test
I can give you some tips to start. :)
From an idea to a viable business, you need to focus on refining your idea with good feedback. Asking the right question is key. If you read "The Mom Test" you will know what I'm talking about.
Your job, in the beginning, should be generating good conversation and identifying your ideal customers. This way you have enough information to help you refine your idea into something that people want/need.
If you are worried about people copying your idea which seems to be an issue I see a lot. Make your product unique to your own experience/expertise or give yourself a time frame (ex. 3 months) so you can move fast with a plan in place.
Please let me know if this is helpful.
You need to do customer development, check out books like "The Mom Test" and "Lean Customer Development". But before your prospective customers will want to talk to you you need to get some credibility. Try to create some content that solves problems that they are aware of, then you'll be able to talk with them.
Read The Mom test by Rob Fitzpatrick. It will walk you through everything. And it's a short read.
If you haven't already I highly recommend reading The Mom Test by Rob Fitzpatrick. He gives clear, simple advice to the entire process of conducting successful user feedback successful. A short brilliant read. Here is a good summary to get out some helpful key points on managing the feedback particularly that I think will help you(go to bottom of that post): https://www.khanna.law/notes/the-mom-test
The Mom Test - https://www.amazon.com/Mom-Test-customers-business-everyone-ebook/dp/B01H4G2J1U/
Everyone is going to lie to you. By formatting the questions correctly, you can to the truth.
Meeting audio - Record them in mp3 format in very good quality. I use the Zoom H4N. It's overkill but works great. Then get the audio transcribed using Temi (https://www.temi.com/) so you can each read and/or listen to the meeting.
https://www.amazon.com/Zoom-H4n-Pro-All-Black/dp/B07WRDLNQZ
Share a google doc for each meeting so each person can add notes about what they heard in each meeting. That can be done real time.
Start your interviews with the people/customers you care the least about. Those will be your worst customer interviews. Really use those to hone your interview skills.
Good luck!
They're more marketing oriented... but I've found these two books to be super helpful in terms of thinking about how I go about asking questions to / requesting feedback from people:
I've found that it's really easy to poison the well, so to speak, so I try to keep it open ended:
If they give me that, then I ask to extend the interaction... just fielding general feedback on the changes I'm thinking about making to the story. Or maybe I simply tell them "I think XYZ is a problem — what about you?"
Everybody has opinions, but I find that keeping it more focused on more concrete things (where did you put the story down?) rather than "is this good or not and why" type questions leads to more helpful stuff.
Actions speak louder than words, so I try to focus on what actions they actually took while reading
Still the best book I've read on structuring your survey / discovery questions: https://www.amazon.com/Mom-Test-customers-business-everyone-ebook/dp/B01H4G2J1U
The Mom Test is an awesome book for learning how to conduct customer interviews. Nice and short, highly recommended!
Product(idea)-founder fit
Before you start, you should answer these questions:
If you answered yes to at least two of the questions above, you can continue with problem validation. More of this in this article.
Problem validation: is this a problem worth solving?
Define the problem hypothesis should be stated as [audience] has [problem]. The problem should be the thing that they're trying to achieve but can't because of some obstacle [1].
The audience definition should give you hints about where to look, know where they hang out to answer the following questions:
Testing your problem hypothesis
You think you've found a problem worth solving. Now it's time to test it. Create a simple landing page (shouldn't take you more than 30 min.) with
Promote your landing page/blog post: post the link in forums, or pay for ads in social networks
Avoid common mistakes like asking people what they think of your idea or thinking that you have the problem is enough validation.
Hope this helps :)
Resources
You can use Facebook Groups or an appropriate subreddit.
But before you start interviewing, I recommend you read The Mom Test: https://www.amazon.com/dp/B01H4G2J1U/ref=pe_385040_118058080_TE_M1DP
It's a great read that will help you ask the right questions. Hope this helps.
I prefer customer interviews, e.g. The mom test (not affiliated) https://www.amazon.com/dp/B01H4G2J1U - you'll be able to capture potential problems that surveys will not.
I have also used Mechanical Turk on a few times but it was mostly a waste (for trying to validate product idea / improve onboarding / identifying new customer segments).