No, you most certainly do not need any malicious intent. It's quite simple and there is no need to factor in any kind of intent: User A advertises product X with 5 Bitcoin and sells said product to user B under this pretense. User B later discovers that said item was never funded by user A and does not contain 5 Bitcoin. How exactly is this not a scam until rectified? An example of this would be Apple selling you an iPhone (under the pretense that it usually does), but actually you just received an empty shell without internal hardware.