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.