Propagating seedlings for sale

Hi there, I do have a nursery for perennial medicinal plants and the like.
But there is a huge demand for seedlings and peopley buy in the big box store otherwise.
I had no problem with tomatoes, chilli and so on but I DO have a problem with everything leavy like lettuce, brassicas. In autumn I tried it and they simply did not turn out sellable. I took care but they came up uneven si some cells were filled some not. They didn't look terribly great either. (For my own use I mainly do direct sowing)
I did grow outside but with automatic watering 3xday.
So what are your tricks to get good looking seedlings for sale and TO SOW EVENLY? I don't want to use fluorescent lights, fans and other expensive contraptions. Our house is too small to raise them indoors and I'm in Australia and we don't have basements!
Thanks in advance!

My trick for getting seeds to come up evenly is to pre germinate them in damp paper towels.  The paper towels are inserted in a plastic zipper bag and I check them every day or so until I see them sprouting. Then I transfer the sprouted seeds to larger containers making sure to give them plenty of space.  After the seedlings are well established I transplant them into individual pots and from there they go to the garden.  

Updated to add that I also pre-soak or pre-sprout many of my larger seeds as well, especially if there will be a few days without rain.  The pre-soak allows the seed to swell and absorb water.  I then plant directly into the garden, making sure to keep the soil moist until the seedlings emerge.  Sometimes I pre-sprout the seeds too.  It just depends on the weather and how busy I am.  I've found that presprouting peas is advantageous as the seeds have a head start and I don't have to worry as much about them rotting in the ground.  This year I was short on pea seed and most seed companies were sold out, so I started the few I had in peat pots and transplanted about a week after they sprouted.  

Using really good seed makes a huge difference as well.  If you use big box store seed, your results will probably be mediocre.

Good points from the previous answers, especially for bigger seeds.

I would like to add that I had great results this spring with sowing in little containers first and then pricking into quickpots. I had a beautiful plant in each compartment and did not have wasted space with empty cells. After pricking I used soil that was mixed with my own compost so that the little plants had enough nutrients in that second stage.

I got inspired by Charles Dowding, might have been this video:
https://www.youtube.com/watch?v=OXR92xHfgic

I presprout bigger seeds which go directly into the garden as well. michelle do you do this presprouting with lettuce seeds to produce seedlings?  I usually sow lettuce directly into garden beds, but people want lettuce seedlings! Someone told me they use a fan over a heated bed in a greenhouse but really I don't have all these things. And I don't like electricity mixed with water...

No, I've never presoaked lettuce seed as I usually just direct sow too.  About the smallest seeds I've germinated on paper towels have been cabbage, brassicas and tomatoes. The simple way would to put a few seeds in each cell and thin all but the largest seedling.  However, I hate the thoughts of wasting those extra seedlings so I'd probably do what Anita does and pricking out the seedlings to their own cell.  

Gary Pilarchik (The Rusted Garden) on YouTube has several videos on seed starting and over seeding.