In theory it is illegal but very difficult to prove. In Europe or within the EU the biggest conflicts in selling wine and price discrepancies lie between importers and french cavistes. One just has to go onto winesearcher and one will see the prices for champagnes especially are much cheaper in France than they are in the numerous countries in Europe. The reason for this is quite simple. In France almost every producer sells direct to cavistes, the producers want their wines represented across the country. This leads to price discrepancies: A wine or champagne will obviously be more expensive in Paris than in Reims.
A really interesting example of how one producers plans went wrong happened at the hieght of the Covid pandemic, at this time we got really good allocations as restaurants were not buying. One big producer got it into his head that he would sell more to cavistes in Paris, who were in proximity to restaurants that sold his champagnes, the caveat was, the had to sell at above 50€. The problem was the same champagne could be got in Reims and on websites for 35 € - 39 €, the parisiens did not buy and the cavistes to get rid of the champagne had to reduce by 15-20%.
The bigger problem is producers sell direct to cavistes in France but for other european countries want on importer to cover the whole land, depending on how greedy the importer is, this leads to conflicts. Probably the most famous case of this was with Clos de Goisses. In Germany for instance certian champagnes are 20 to 30 € more expensive than in France. In Italy it is more extreme. Customers see the prices on Winesearcher or visit the Champagne region and see the massive discrepancies. They complain to the importers or those working with the importer. In turn the importer complains to the producer.
For many producers the french market is the most important and the french are in general not willing to pay such inflationary prices, some ignore the importers others more dependent on foriegn markets have to take heed. Larmandier Bernier was definitely pressured by importers to increase, I know his champagnes do not sell in the Champagne region, too expensive and enough alternatives.
Another good example was Tarlant, you could get the Brut Zero for under 30 €, the bigger champagnes for 60-70 €. Through pressure from importers they pushed the prices up, the Brut zero overnight went to 40 €. A caviste in Beaune told me none of his customers will pay 100 € for the Crus which he previously sold for under 70 €. There was no quality increase.
The best example is Lacourte-Godbillon, a nice producer but nothing special. In contrast to their nieghbours, Savart, Maillart or Emanuel Brochet, they have a long way to go. Up until 2019 they were pretty much restricted to the french market. the Terroirs d’Ecueil cost 25 € and the Mi-Pentes 35 €. In 2019 they started working with a german importer notorious for high prices and the trouble started. The importer’s recommended selling price was 36 € for the Ecueil and 50 € for the Mi-Pentes. The divergence between the french prices and the german prices are quite big. Lacourte than increased their prices for the french market and no surprise the cavistes in the Reims regions stopped buying.
About six months later they realised the drop in sales in the Champagne region and asked cavistes what the problem was. They did not like what they heard.
Italy I believe the situation is much more extreme, a client told me, there are 4-5 importers who sort of dictate the prices which are really high. Before Brexit the english were probably the biggest customers in the Champagne region, this broke away, now it is the italians. Restaurant owners, cavistes and private customers are making the trip up to the Champagne to buy as it is much cheaprer even buying at retail prices than buying from the importers.