When learning about Ona'a (ShuA CM רכ"ז) I notice, that despite the long list of Halochos there are no measures of prevention mentioned either for the seller or the buyer(as far as I could see).
I could think about an obligation to check the prices in advance, to stick price labels on all products etc.
I think of myself as G-d fearing guy and I'd like to beware of Ona'a when selling my home-grown tomatoes at the local market (Shuk Ben Yehudah in Jerusalem), for example. (I'd like to make my example as close to their times as possible).
It should be noted, that many Poskim say that איסור אונאה and דין אונאה are two different things, and איסור אונאה (the prohibition) holds even for less than 1/6 it's just cannot be claimed (for making Teshuvah on that for example). So I'd like to be very careful about it even for less than 1/6.
What should I do when I come to the market, how do I care not to transgress this important Mitzvah?
-Should I make a survey of the existing sellers before I set my price? How often should I do it throughout the day? Should I contact the local Rabbonus, the Ministry of trade or other authorities?
How can I compare my produce to others' to set a fair price? How different my tomatoes can be to claim it's of a different sort, organic, 30 days in the fridge warranty?
What should I tell every customer about the history of the tomatoes and my business model - how much I spent on them, how much I plan to profit and why my price is different from others'?
Similarly, if I am a buyer and I come to the market for tomatoes, how can I be sure I don't override this Mitzvah when buying cheaper or more expensive than "needed"?