I've read some people are accepting debit or credit cards but this seems risky as well since they can call their bank to dispute the charge.

So I have been mostly using my brokerage account to buy properties as my credit line.  

I am about to start the process of shopping for conventional loan rates and getting pre-approved, but am concerned if I shop too much within a short time period it will adversely affect my credit score.Does every loan inquiry during the shopping process negatively affect your credit or is there a "grace" period after the first inquiry?

I'm independent, no debt, no student loans, very low tuition (with financial aid grants), rent of $500 per month, credit score above 750 and a small chunk of change saved up in the amount of $15,000. 

Personal loan via credit Union for expenses4.

Another avenue would be to use some of that 150-200k to pay down your mortgage, then take out a line of credit on your home to get started in the Real Estate Investing world.

The benefits are compounded.  

If you purchase a property in the Midwest for $30k at a "20 cap", and you get lucky that CapEx doesn't kill you, and lets assume that you earn $500/month of true cash flow even after 10 years of holding (this would probably never happen, but I am giving your statement every possible benefit of the doubt).Or, you purchased a $500k property in Northern California at a "3.5 cap", this was bought at a true market cap -- no discount.

I choose the tenant based on credit and rental history without any set ratios as the quality of applications varies from one neighborhood to the next.

If so, sounds like a good way to leverage your benefits.