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.