this error comes from the underlying library libCURL that handles the (FTPS) request to your server.
libCURL is OS level, correct? If that were the case, wouldn't any software that uses the same library (or even just using the terminal FTP commands) experience the same error when uploading?
Can you increase the timeout on the server for such connections? That may help, but only if the connection timed out on the server.
After a fair amount of experimentation I think I figured out the issue, or at least the cause.
If I start an auction with ONE image, it works fine 100% of the time. If I start an auction with two or more images, I sometimes get this error. When the error does appear, the first image is always uploaded but the second image doesn't if it's "too large". I haven't figured out the exact numbers but when the second image is tiny, like 50K, it seems to work all of the time, but any larger than that seems to generate the error. When there are three or more images it always fails; again, the first image is always uploaded but any/all images after that fails, regardless of size.
I've seen this set of conditions before (like a decade ago) and the problem was always the same: too many simultaneous connections on an FTP server that doesn't support simultaneous connectins. The problem seems to be that GarageSale is opening multiple simultaneous FTP connections to upload the images. If there are five images then GarageSale opens five simultaneous FTP connections, but if the server doesn't support it then it fails. One image is fine; there's only one connection. Two images works IF the second image is tiny; the connection (connect/upload/disconnect) happens so quickly that it's faster than the FTP server's "reaction time" to the second connection, but if the image is too large then the server kills the connection. Three or more images is right out; the FTP server kills the others immediately.
It's easy to point the finger at the server and ignore the problem, but fundamentally the solution is simple: Put an option in GarageSale that forces single FTP connections. If there are five images, instead of having five FTP connections have a single connection and run all five uploads through that single connection. Yes, it'll be slower, but it will also work and that's more important, IMO. I believe FileZilla has such an option and I definitely remember seeing it in programs such as Dreamweaver, so it's not like it's an impossible or even particularly difficult thing to do.
I would very much prefer to maintain complete control, ownership, and permissions over my images, so using a third-party "solution" isn't really viable.