again I say no, as what you are asking is for anet to sell achievements. All mastery points are tied to achievements, even the insight ones that are basically free points in the maps. Levels are not tied to achievements neither are the waypoints. if you can not get the hard achievements then don't go for them and just focus on the easy points or go for the points that you are willing to put the work in for.
The only masteries that actually block any kind of progress only cost 1-3 points if that and are at the very beginning of any of the mastery lines. Which you get the points for by either doing the story, the insights or just given to you by exploring the map. So all of those are basically a give me as you can get them extremely easy and unlock them real fast. All other masteries after those initial ones are secondary and are not really required to do anything else. So by you wanting Anet to sell mastery points, which are tied to achievements, means you want them to sell achievements in this game. Whatever happened to players actually playing a game to achieve a goal in the said game?
So should mastery points be sold in the gem store? No, as by doing so, would be selling achievements aka something that is achieved by performing a certain task. This would be like asking Anet to sell legendary armor, backpiece, or a legendary whatever on the gem store. It would be a slap in the face to those who actually put in the effort to "achieve" these things. So selling any of the mastery points, would be a slap in the face to those who put in the effort to achieve that particular mastery point in an achievement.