I sow a good number of seed each year, and don't need all the seedlings that germinate, so I sell them. Typically, I would not sell them until they are 3 years old, and go into at least a 6cm pot. In some species that can take longer than 3 years, for others, 2 can be enough.
I have found that the most demand is for species that flower early and have large flowers, so look to the Ancistracanthae Series, though they aren't always the easiest to grow at the seedlings stage.
I don't know which country you are in, so propagator brands vary a lot. You will, for best results, need a thermostatically controlled propagator, with good lighting above, and I have found I get best results by using the "baggie" method - sealed plastic bags into which the well watered seed pots go.
I built my own propagator, buying heating cable and a thermostat, and this is by far the most economic way of doing it, and not too hard if you have basic woodworking skills. There are a number of website which tell you how many watts/sq metre you need to raise an area to the desired temperature.
Here in the UK, I bought my cable and thermostat from Parasene, and made mine to fit a 52cm commercially available preformed plastic lid from Stewart. That lets me have 2 standard seed trays sitting on the sand with 24 2" seed pots in each.
I usually make 2 sowings a year, late February and May, though I have tried a later one, but the seedlings seem to struggle more during their first winter - probably my fault!
I tend to sell them at meetings, local Branch or when I go to other branches of the BCSS, at Shows and of course at the Mammillaria Society Annual Meeting, which frankly is only really accessible for members who live in the south of the UK, as we hold it at RHS Wisley. I have also listed them on the BCSS Forum, and on this Forum as well, though not for a few years.