I also have a close friend who engage in petty trade, the only record she keeps that i know of is the debt record, because she have different people buying some of her items on credit so she only bother about keeping records of her debtors. I guess by doing this she can already be able to know her cost, expenses and profit.
Keeping records is very important but some business owners may sees it as time consuming and may not be interested in doing so even if it will be very helpful for the growth of their business. This can also apply to none business owner, anyone can keep record of their daily expenses which can help them to plan properly.