Lets be real here for a second.
Customers come to this market (or any 3D market) to buy quality content. So you as the Vendor should be reciprocating - meaning purchase some quality materials/textures! If you invest in your craft you will earn more from your craft. Not only do you support the artist who spent a great deal of time making these textures/materials, you support your own growth making quality 3D assets for your customers. Free textures are a good starting point to learn from but at a certain point you must start to move forward in this field and provide your customers high quality products so they save their time, you build a better reputation and you enter the win-win system.
Give your clients the best quality products you can, buy high quality textures. Its works the same way you do, if you purchase some great fabrics/woods from an artist they earn money from their work, you use these in your products in your assets then you elevate your work, make more money and in return the customer also has a better 'render' for their client, its win-win-win. Since you purchased those assets you can re-use them for other projects/products 10x over if not more.
From your initial question, did you want some resources/websites that provide high quality textures for your assets? I have a list if you want and can provide them here, tell us your intention.