This talk from Dalia Lasaite reveals some key insight about the 3D market and also outlines some best practice on how to go about selling 3D stock (https://www.youtube.com/watch?v=xI_HpXuPWms)
Keep in mind when a buyer gets confronted with multiple options they will go for the one that shows most promise. For example your AR15 listing, if people types AR15 in the CGtrader search field they get to see a list of top level fully textured highly detailed AR15 models, some for 5 bucks. How is your model exceeding those listings, what advantages has your model over those? The answers to these questions should be outlined and reflected in the previews and descriptions so that a customer gets convinced yours is what they need.
Also try make things that are not there jet, in that regard the carl gustav probably has best chance getting sales.
Also remember your basically selling via the images so renderings need to be extremely good. Your rendering skills need some technical improvement regarding lighting and gamma setup because the gamma levels in your images are not correct. Also note when renderings get very good and close up then the model needs to be holding up or its flaws will reveal. If the model is very good then it will show in the close up images and then you get customers attention. On other hand, if close ups are avoided then customers get suspicious because they cannot properly determine the quality of the model (do not get to see how detailed it is).
Try to be the most demanding customer possible and ask yourself what you want to know when purchasing models. Probably you want to know what type of textures there are and resolutions they have? Are there multiple materials on the model or is it a single PBR material? Maybe the model is in parts or it has material ID’s?
Just be as detailed as possible about everything the package delivers.
