Once upon a time the independent lens makers produced low-cost alternatives to Nikon, Canon, Leica, Sony, Contax et al.
That time has passed. Sigma in particular has now switched its focus to high quality alternatives that, in some cases, perform better than the lenses produced by the camera manufacturers.
Take the Sigma 50mm Art as an example. The optics are absolutely fantastic and it will hold its own against any lens built by any manufacturer. It is a little on the chunky side but that's a fair price to pay if you want top notch performance.
Please remember we're always in the market to buy high quality equipment of any age. We always pay high prices and can arrange collection if required.