I know that FX operates that way (that´s the main motive I stay far away from it, and recommend everyone to do the same). I didn´t had any suspicions about equities because before everything was centralized and they had a lot of regulating entities who watched the business. But lately I started trading in a more technical way, and some doubts arose about it, and wanna check this. BD cannot manipulate the quotes themselves as in some of the FX worst cases that I heard of. But getting an execution when it wasn´t suppose to happen, or being unable to help a level get trough can really affect the trading process.