There are different definitions for different purposes, like for taxes for licensing, to prevent charging for certain services without a license. etc. About twenty years ago, our local district attorney, (long now retired) was prosecuting a politician who was caught paying for sex. I'm not sure what motivated the statement, whether an unthinking moment, or two many drinks at lunch, but at the sentencing, he made the statement, " we all pay for sex, whether for cash, or dinner and a movie" There were nationwide jokes about how suddenly a woman who went on a date where the guy paid, and then had sex, was a professional whore and should be arrested. .I remember specifically Jay Leno making fun of the stupid statement. Something to the effect, that if a wife says, Honey is you stop and get a bottle of wine for dinner, you'll get lucky later." Don't do it guy or you will be married to a prostitute. ( On the other hand, Why the prosecutor said something to help the defendant at sentencing was another issue)
Under the uniform commercial code, they look at the matter somewhat differently. The rights and liabilities to certain contracts vary depending on the status as a consumer/non merchant or merchant. A person who is a merchant is one who deals in goods of that kind. It does not even have to be for profit and depending on circumstances, might even be giving the merchandise away for free. A person who swaps and horse trades without money transferring can be a merchant Certain warranties and liabilities attach to transactions whether or not money is involved. .I once bought a truck from a guy. I saw the truck for sale in front of a house with a price sign in the window. The transmission died a week later, and the transmission mechanic said the transmission had recently been worked on because of certain new shiny bolts in the housing. Well I went by that house every week for six weeks. There was always a vehicle sitting there with a for sale sign or price sign and always the same number. I took pictures each time, and then filed suit against the guy under the Maryland law pertaining to vehicle warranties. he screwed up and did not write as is on our hand written contract. I collected $2,400 to put a new transmission in the vehicle. I could prove that he came under the definition of merchant under the law. he "dealt in goods of that kind", ie. bought and sold vehicles as a side business. .
Suppose a turner makes some salad bowls and uses a tainted finish on the bowls and he sells a dozen or so in more than one sale in a year. A person gets sick due to the finish and sues him for breach of warranty of fitness under the commercial code. he has made the bowls and made multiple sales. He is probably a merchant, making him liable for the illness regardless of any fault on his part. .