Quality, Complexity and Demand. These are intrinsically linked.
Quality - People want good quality assets, attention to detail to mimic the real product. So screws, bolts, hinges, chamfer/bevel details etc for modelling. Realistic materials/textures, uv's should follow the correct direction such as how wood is warped in the workshop and showing the wood ends where the wood is cut, non repeating textures etc. For example your "Window box" is missing all these elements.
Complexity - Customers want value. Value is time saved + quality. So if you can spend $50 on a model and save yourself 16hours making the model then its certainly worth buying. This also ties into quality, meaning if I buy an asset that looks great and then have to spend time fixing materials/textures this is not good value. It should look good in all lighting situations with minimal adjustment required, if any. Models made with simple textures and minimal geometry detail will never result in a high sales volume. Remember the people buying your work are also 3D artists so make it worth their while.
Demand - Each industry has a trend. For example if you work in archviz then you work closely with interior designers and know the common requests. Same if you work in games, usually there's a bunch of assets that are always needed for every job and variations of these assets can make their team work faster or more efficient. This comes down to market research. If you are savvy then you can find these pain points in forums where people are asking for help, but that's a bit of a hit and miss in my opinion. My advice is you need to get your hands dirty, do the jobs they are doing, feel the pains they feel and then fix them. That is the job of the content creator.
