I own a small skin store so hopefully I can provide a bit of insight for what has helped for me. For shapes:
If you have ONE shape for a specific skin, it's optimal to have the vendor next to the Skin. That way, if people like the skin but do not feel like it fits them well they can visible see that they have the option of buying a shape that was designed to fit the skin best.
Also, ALWAYS have a DEMO. I cannot stress that enough. It's wonderful as a consumer to have that option so they don't feel like they are taking a risk by buying blind. It also saves you the owner the possibility that someone wants a refund because they didn't like how the shape ended up looking.
If you have multiple shapes for one skin, I still would advise having at least one shape by the vendor. Then you can set your other shapes in another section as other possible options. But please have a DEMO for those as well
For appliers: This is kind of a tricky area. In general, packaging all the appliers with the skin will make people happy but it also can be a lot of extra work/stress for you depending on how many you end up supporting. In addition, you may choose to bump up the price by including appliers which may deter some people from purchasing because now the price is a lot higher and they may only want one or two appliers. One thing to take into consideration is whether or not you plan to change the body of every skin from one release to another. Or, keeping them the same because then this will change how many appliers you end up having to do per release (and those take a lot of time!).
In the past, I've sold appliers separately by skintone. The bodies of my skins stayed the same so customers could pick or choose which applier they wanted and apply it across various faces if they ever decided to change things up. However, I recently updated the bodies to my skins so now I've been packaging the new appliers with my skin releases. I sell appliers now in "Packs" : https://www.flickr.com/photos/mochimilena/15311518574/
Each pack is priced different depending on what is included. This option allows individuals the flexibility again to decide what they really need without feeling like they paid too much for "extra appliers" that they may not want or need.
Hope that helps!