The one caveat of team buying is that players will almost never get what they're paying for. The irony is that those who purchase teams are almost never equipped or skilled to compete at the level they're purchasing. These players often end up tanking their newly-bought team a couple of hundred points just to complete the minimum 10 games to qualify for Arena point gain. In this way, team purchases are an unwise investment unless players can competitively maintain the team's rating. In some dastardly cases, very high-rated teams are bought by win traders who use the purchase to inflate their team ratings.
Another common way to sell Arena points is through selling team spots. Usually done with 5v5, where the selling team can exercise more control, point sellers rotate up to three (more if they're confident with their set-up) buyers on the fifth spot, allowing each one minimum three-game participation. Limiting games played to the requisite ten permits little variation on the team's rating. These purchases, or team slots, go for about 75% of the Arena points the team guarantees, e.g. 750 Gold for 1000 Arena points. Sometimes arrangements are made for permanent spots on a point selling team. In smaller team brackets, such as 2v2, the point selling team will usually play 7 games and leave the remaining three games under the control of the buyer.
Currently, Blizzard while does not support Arena point selling, it also won't punish it. However, Blizzard cautions point sellers who attempt to scam buyers, even though their general stance is caveat emptor. They have stated on several occasions that it is not a punishable offense, but win trading -- which is considered an exploit of game mechanics -- is. Win trading is sometimes used by some point sellers to artificially inflate a point sell team's rating. While it used to be possible to purchase titles and Nether Drakes, it has become more restrictive -- though not impossible -- with the introduction of personal ratings. Because personal ratings reset to 1500 when joining new teams, end-of-season rewards are slightly more difficult to attain because of the point discrepancy requirement stipulating no more than a 100 point differential between personal and team rating.
Is Arena point selling detrimental to the game? Blizzard's stand on Arena points and Gold is that they are in-game items and that trading them isn't unlike selling goods or services for Gold. What does this mean for the Arena tournament as a whole? To be quite honest, not much. Dedicated Arena players will climb up the Arena ratings as their skill and gear allows, regardless of point selling teams. In some cases, teams may even benefit from sold teams as they tank through their minimum ten games. If anything, the prevalence of point selling devalues Arena gear, and in a roundabout way contributes to leveling the playing field. The more epic level Gladiator gear gets distributed to the player base, the more skill and team synergy becomes a factor to winning. On the other hand, Tom Chilton revealed that Blizzard is going to address point selling, so there might be more changes to Arena mechanics soon. For the meantime, point selling is a mere means to an end.