Some investors walk away with a false sense of security and a lot of unnecessary overhead work and costs. 

I know this is not a traditional method but it works for me because to collect that money up front would pay for my holding costs for 2 months regardless of wether or not they come up with the rest.  

Can you guys share any unique method to offer the seller without insulting with low ball offer.

I've personally used the BRRRR method by buying low enough, value add, and refinancing. 

You mentioned a promisorry note but you did not mention a mortgage (or deed of trust) to secure the note.

Managed to secure a pre-approval that can allow me to purchase up to 350k with 25% down payment.

But, as always, an attorney well versed in securities law is going to be your best asset.

One concern I have is the tenant turn over process and how best to handle the inspection (security deposit), cleaning, keys management, and potential showings for new tenants.

What kind of platforms or methods are successful in finding deals as a wholesaler?

The first step is to secure a lender that will allow you to use a secondary source for coming up with the down payment.