I think a vendor now needs to cater to both Poser and Daz studio even if it is only supplying some Daz compatible materials.

A broker is a good start, since they can help with product testing, and help tidy up readme and advertising material but I dont know any site that actually willproduce an advertising image of a product.

Running your own site can be very time consuming and has its own issues with ecurity, but a group of vendors may be successful working together. One modeller, a couple people doing conversions to Poser, Vue, Studio , and a texture artist may be more productive than doing it all yourself.

You need to find a unique niche though. Summer frock 1,956,728 is not going to sell well compared to Nipple Morph Pack 1. 

It can be a good supplemental income.