Do I need a business license to operate a garage sale if it is profitable and a significant source of income?
It would be worth speaking to the local town clerk or a business attorney to see if there are any issues concerning the amount of money earned and if there's a tipping point that requires more substantial registration.
Your reselling of merchandise also raises some concerns--it's unlikely that the merchants would want to give resellers 90 percent discounts on their merchandise; they'd rather give that to the end-user customers as a means to build a loyal customer base.
Your all-cash/no receipt/no sales tax policy could also run you afoul of the taxing authorities, as you really are treating this like a business. Speak to an accountant to make sure your bases are covered.
Nina L. Kaufman, Esq. is an award-winning New York City attorney, edutainer and author. Under her Ask The Business Lawyer brand, she reaches thousands of entrepreneurs and small business owners with her legal services, professional speaking, information products, and LexAppeal weekly ezine. She also writes the Making It Legal blog.