As Sale is, in practical life, the most frequent and important contract, it is not surprising that it figures largely in the texts in connexion with slaves, and is the subject, in that relation, of many special rules.

Slave-dealing was a recognised industry, carried on, apparently, by men of poor reputation. It seems to have been on account of their tendency to fraud, which they may have shared with dealers in cattle and horses, that the Edict of the Aediles was introduced, with which we shall shortly deal. As being men, slaves were not included in the term merces and thus slave-dealers were not mercatores, but venaiiciarii, their stock being called venalicii. Where slaves were so numerous, the traffic in them must have been a most important industry. There is indeed plenty of evidence of this, and of the fact that it was often carried on on a very large scale. Wallon gives a lively account of the usages of this trade, of the tricks of the dealers, of sale de catasta, and of other similar matters, too remotely connected with the law of the subject for mention here.

Such a business would require large capital, and thus it was frequently carried on by firms of partners. A text of Paul, speaking of the practice of these firms, says that plemmque ita societatem, coeunt ut quicquid agunt in commune videantur agere. The sense of this is not altogether clear. Though expressed as an understanding among themselves, it seems from Paul's further language to have been treated as affecting outsiders.