How are you using the command prompt to post the listings?
It could be an error in your listings or due to the API itself. If you have been posting the same way with no problems, I suspect that the error is not on your part.
It cant be my end, cause my brother and my friend are also having the same problem. This happened yesterday when Gameflip updated a few things on the website.
I figured out what the problem is, instead of uploading the image of the item itself, i was using a image URL address, don’t know if its a problem with Gameflip or the website where i got the images URL links from but i will definitely post back on here once i figure out.