According to IRS, purchases, sales, payroll, and other transactions you have in your business will generate supporting documents. Supporting documents include sales slips, paid bills, invoices, receipts, deposit slips, and canceled checks.

Specifically to your question, you should keep a record of each purchase and sale in the course of your business. If you are using a credit card for purchases, the credit card statements qualify as satisfying the record keeping requirements. You ay be using a merchant processor to accept credit card as payments for your sales, that will satisfy the record keeping requirements for sales.  Any online account that you are using to sell will have a list of account transactions and that is a way to satisfy record keeping requirement.

Basically, anything that can provide details of the income and expenses incurred in your business is qualified for record keeping. 

Documents for gross receipts include the following:

• Cash register tapes
• Deposit information (cash and credit sales)
• Receipt books
• Invoices
• Forms 1099-MISC

Documents for purchases include the following:

• Canceled checks or other documents reflecting proof of payment/electronic funds transferred
• Cash register tape receipts
• Credit card receipts and statements
• Invoices

What kind of records should I keep