I'm pretty sure that Bungie controls what he sells in advance. The data is available ahead of time to look at (in the network's data packet lineup), but unaccessible to players until his reset when he arrives each week. It may still be RNG, but the chance of that is low with the other information in mind (if it was predecided a week or two in advance, the likelihood of RNG itself is low) That's the current theory, anyway. But, if it were decided through RNG, specific pieces of the DLC equipment would probably have a higher weight, but might also have rules in place to prevent them from appearing more than once in a set amount of time.