Somebody recently stole something from me, posted it online for sale, and then took down the ad. Now obviously I managed to find a cached version of the ad, but my question is, is there any way to find the IP of the person who originally posted the ad?

Probably not. Most places people would post ads on the internet, you would not be able to get the ip address of the poster. For example, trying to get my ip address from this forum post is not going to be possible for a normal user of d2jsp. Of course the site administration personal have access to this information, but they won't give it to you without a pretty damn good reason (they would probably require law enforcement/court order).

That being said there are lots of ways to get someone's IP address. But I think I should ask: why do you want their IP address anyway? You can't get someone's physical address or name from their IP address without help from that person's ISP (which again is going to require the help of the authorities). All the IP address will really tell you is the general area they live in, like "Los Angeles" or "southern Michigan", and that they use Cox Cable or Verizon dsl.

This post was edited by Azrad on Sep 16 2013 12:48pm

I'm looking to match the IP address of the poster of the ad to the actual IP address of the person in question. I know who it is that stole from me, but I need proof that it was actually them who posted the ad and not somebody else.

Well one cheesy method of getting someone's IP address is to send them a message that contains a hotlink to an image stored on a server you control. When their browser tries to draw that page, it will need to download the image from your server. Your server logs will then have the IP the browser is using. If you sent the hotlink to them in a private method (where others can't see it), they will be the only person downloading the image from you and then there will be only 1 IP address in your log.

Yeah, thanks man, I was planning on blatantly confronting him through Facebook about it and getting him to click the link of his "ad"

But if this is a serious theft, I would just call the police. If they take it seriously, they have the clout to get the IP addresses from servers, and to get the info on who was issued that IP during the time period in question (from the ISP).

This

Also yes if you just want to grab their ip it's fairly simple

I used the following website to grab the ip of the fake "jessica nigri" on jsp
http://jessicanigriblog.netne.net/

You don't need to go there if you don't want, it's just a working example, the following code is what is used on that page->
Along with some text to scare him

This is pretty hilarious.