How much is my Tau army worth?

It really depends on the quality paint job. Half of the warhammer 40k hobby is putting together and painting your models, and the best way to do that is just by the models from GW. However, if the models are quality painted and based. you can get probably about 1.5x box price of the given model. But it's a two edged sword, as poorly painted or unpainted models go for less than box price (ive seen anywhere from a few bucks less for unpainted to half price for poorly painted). I would set the starting price as a little less than box price for your models, about 80%, and see where it goes from there