There are several distinctions and differences between a submissive and a slave. I will take the term submissive first. A submissive is a person with an intense desire to serve that does so under some conditions. The conditions would typically include terms of service, length of service, which areas of the submissives' life the dominant does not get to control, what the hard and soft limits are when they play or just interact, and of course the safe word. In many ways the submissive controls play. Hard limits are activities that a dominant will not be able to indulge with the submissive. Hard limits broken would lead to the end of a scene, relationship or arrangement and a serious breach of trust. Soft limits are limits that can be broken, but only after it has been discussed with the submissive. These are things the submissive is not excited about doing, but will try if it seems to excite the dominant or causes some latent excitement and fear in the submissive as well. The safe word ends all play. The dominant has free reign as long as he or she stays within the set limits.
After a scene, many submissives go back to controlling their own lives. They slip in and out of the role of submissive in most cases. They continue to make their own decisions and even in a 24/7 situation will continue to control those areas of their lives that are off limits to the dominant. A submissive submits every time a scene starts or some activity where the dominant has been given control commences. The choice is hers or his every single time the dominant requires submission. A submissive can walk away from a scene because something is not working for them and completely slip out of role whenever they feel the need to discuss something about the situation with their dominant. This does not indicate a lack of the need to submit or serve though.
Now we turn to the slave - the consensual slave. The slave gives up all rights to make their own decisions, becomes the "property" of a Master or Mistress, takes on the limits of their Master or Mistress and does what is asked of them regardless of their feelings about it. Sometimes a slave will give up all rights to property of their own and will continue to work for the household, having to ask permission to use any money they earn. Slaves earn privileges and do not have rights. A slave submits once - when the collar is placed around his or her neck and when something is difficult, has to ask for help to accomplish it. Slaves are not kept in basements or locked away forever. They are strong people who have an intense desire to please another human being and now has the freedom to live that desire and not have to submit to anyone else but those chosen by their Master or Mistress. Taking on the limits of another means that slaves have to ensure that the limits of the dominant they are talking to matches theirs, as their only choice is choosing the dominant. It is responsible to make sure that the person does match them or their preferences.
Can a slave be sold to another? No, most Masters or Mistresses would never do that. Remember this is consensual slavery, a 24/7 arrangement with one person fully in control and another person obeying all the commands and wishes expressed by the other because both of them want this. There is often a lot of love involved in this type of relationship as well. There are mock slave auctions that are used as fund raisers in the community, and slaves would go and serve another man or woman for the evening, but they always go back to the person they chose to be with. Can a slave end a relationship? Yes, they can petition their dominant for release and no responsible dominant would say no if nothing could be done to heal the relationship.
The difference between the two does not make one better than the other. I have seen countless submissives that serve as beautifully and perfectly as some slaves do and prefer not to be called slaves because of the negative connotation to that word. I have also seen many wannabe slaves struggle until they eventually found their place as submissives. It is difficult to devote one's entire life to another person but it is also incredibly rewarding.