What is this plant I saw for sale at a Romanian farmer's market?

That actually are tips of trees, probably spruce¹. It’s this year’s new growth and a very seasonal product. The tips are harvested when the are (about) fully grown, but still light green and soft.

I can’t say for sure what the local customers would use them for, but one of the common uses in my area of the world is to make “honey”, i.e. a syrup by either layering the tips with sugar and let the sugar draw out the aromatic compounds or alternatively make a “tea” that’s them preserved with plenty of sugar and boiled down to a syrup. It’s both a honey substitute (it does resemble fir honey) and a folk medicine against colds and coughing. Other extracts are made with alcohol instead (or in combination with) sugar to create a liqueur, again served both for medical and culinary purposes.

¹ This is not a botanical id. Various members of the genus Picea (spruce) and Abies (fir) are used, usually whatever grows locally and has a pleasant taste.

I'm French, and I happen to know someone that uses this kind of spruce tips to make alcohol.

It's well-known in the Alps region of France. People tend to use these things to make "sapinette" (which means "small spruce"), a liquor appreciated by many peoples. They let the spruce tips extract their flavour in a prepared-in-advance alcohol.

As for the final result, it most likely has a taste very close to spruce honey. People may like it or not. Its final degree of alcohol is around 35 - 40°.

They are called 'fir tree buds' ("muguri de brad") - young tips of branches. they are used to make a syrup that is believed to have health properties - antibiotic, antiseptic, metabolism stimulator, etc. (pic taken from this link: https://www.realitatea.net/sirop-muguri-de-brad_1939056.html )