When it comes to income and taxation, there is a lot more grey than you think.
Technically, ALL income for business-earning purposes should be reported and taxed accordingly.
Regarding taxation of the buyer, at least in Canada, GST is generally not applicable until the seller is earning $30K+.
But REALISTICALLY - no taxation agency really cares about 1 chair.
I probably wouldn’t consider it a black market/underground economy unless it was significant and ongoing. E.g., your neighbours REGULARLY sell chairs for profit and don’t report their earnings.