You can take the 8 bus there (it should say specifically Mercado Central). The journey takes about 90 min. I went there several times last year, I don't think it is unsafe to go there, at least not unsafer than other places in the city.
There is an area in the back where you can buy retail, outside there are fruits and vegetables and inside meat and other stuff. The trip makes sense if you plan an Asado for a large group of people. Without a car it is however a pain to carry everything in the bus.
