Don't be crazy and you won't attract attention.
If you attract attention the ministry can and will send out a inquiry document to both parties, sometimes months later, asking you to verify the amount of the sale, so you'd best have your ducks in a row. I got this exact document once when I sold an old trailer for (legitimately) a few hundred dollars and wrote the receipt accordingly, but apparently someone thought the value was too low and sent out this inquiry to both parties in presumably an effort to make sure one didn't say one value, and another saying something drastically higher.
We emailed each other again to confirm the values, filled them in, sent them back, and that was that.
Beyond that, unless they find a discrepancy on that sort of paperwork, or they have reason to believe you are intentionally trying to defraud the system (like doing something completely insane like telling them you're selling a 2016 bike for a declared value of $250, for example) I don't think there's much they can do, or will be bothered to do. Just be reasonable, it's not worth a potential fraud charge (as in my above example where it's clearly batshit crazy) to save a few hundred dollars on taxes.