I've never collected portulaca seeds. I opened up one little pod (??) thinking seeds were inside but when I opened it up, it was a little flower. So do you know when it's seeds or a flower ready to bloom?
I have never collected seeds of portulaca, but i noticed that they come back every year.
When the seeds ripen, the seed pod is quite dry and brown. Looks like a little pointy elf hat. You pretty much just lift it off.
If it's greenish, it's probably a flower.
Hope this helps.