Can I automate ad posting on Craigslist using the Fake browser?
After snooping a bit on the Craigslist-form, it seems the IDs are randomized, as you said, and there is definitely a reason for this (to avoid spam). I'd say Craigslist most likely have pretty secure websites and forms to prevent spam and scams, but you might be able to get around it if you were experienced with JavaScript.

I see Fake can run JavaScript, so if you have some experience with that already, you might want to try a follow-up question on Stack Overflow.

As a side note, I would advice against doing this, Craigslist have their reasons for making it harder for you, and nobody likes spam! :)