Typically, you will see loan products that are marketed up to 75%, MAYBE 80% LTV.  

I found there was loans taken out using a 3rd party that was disguised or hidden on their taxes.

You can also get a personal loan up to $50K which typically is to pay off credit cards debts, do renovations etc, but it can also be used as a down payment.

SorryCheck like a 203B loan

Not really necessary.You will need a commercial loan refi with higher interest and maybe more down to cover it.

Here are some reasons not to sell:- 2.7% loan is very low by today’s rate.  

Also wondering if I can get a second loan (Commerical) looking to go bigger with a 5+ unit building using the 75% of profit from first one. 

Get the cost analysis and blue prints done and work it out with the construction loan committee and start the work draw phase.The other options which can be a little easier is buy a home that will pass an Inspection/appraisal and not be "Subject to" but does need a lot of TLC/renovations and do it yourself.

If the buyer got a loan from a traditional lender to get to the 75k number, would that automatically put me in a second position on my seller financed note?

Refi to get a better rate is good but have to weigh the cost of money and the time you have the loan to see if it is worth the costs to get a new loan.