I think of négociants as being particularly important in Burgundy partly because of my lack of detailed knowledge and partly because the Burgundian négociants do such a good job. In other regions I'm more comfortable with learning the names of good winemakers.
As a translation, I'd vote for "merchant" Alex. My impressions are informed by Jancis Robinson's 2d, which provides:
"French term for a merchant (see merchants) and one used particularly of wine merchants who buy in grapes, must, or wine, blend different lots of wine within an appellation, and bottle the result under their own label. the appellation contrôlée system, cheating requires real ingenuity. ...
The role of the négociant is particularly worthwhile in Burgundy, where the négociants are concentrated in Beaune, and where so many individual growers produce tiny quantities from each of a number of different appellations. The selling of the domaine bottled concept by the likes of Frank schoonmaker and Alexis lichine was so successful that it cast a slur on the work of the négociants by imputation. This was unfortunate, but the likes of Drouhin and Jadot have worked hard to prove how they can be a source of more reliable wine-making skills than all but the top one per cent of grower wine-makers. The Burgundy négociants have been acquiring increasingly significant vineyard holdings of their own, so that Bouchard, Père et Fils in Beaune and Faiveley of Nuits-St-Georges, for example, are two of the Côte d'Or's most subtantial vineyard owners. ... The term négociant-éleveur implies that the négociant oversees the élevage of the wine it sells.
Like all important French wine regions, Bordeaux also has a great concentration of négociants, many of which own châteaux (while some of the first growth châteaux also now own a négociant business). "
In any event, many thanks. As always, I learned something new from your post.