I've read quite a number of threads the include a link to a photo on ebay or other site with temporary longevity. You should be aware that the link to the photo is only good for few weeks and someone reading your thread comment after that won't have visual access to what you're talking about.
If it's an eBay listing I don't think you're supposed to just take their original photos and post them. But, there is no problem with taking a screenshot of the actual listing and posting that.
I have a subscription to a site that I use to store any photos that I intend to link to. That way they're there until I find they are no more use.
