I was having that problem earlier today too.
But it was on a Board, not the Sell Your Item Advanced form.
For the SYI, I usually type up the description on a desktop text document, add any coding, and then cut and paste into the SYI.
Then I can also keep those descriptions (and pictures) in folders on my desktop as backups.
I'm a block from a high school and posting during school hours can be a tad slow, but right now they are on vacation.
My Mac is about 10 years old, old enough that I have replaced the keyboard because I worse some of the keys right through. My browser is Firefox.