Alexis, you can send someone a private message here by just clicking on the icon under their name - the OP of this thread hasn't been active on the forum for over three weeks and is probably not going to see your post (although as their website hasn't moved on since they first posted their ad and you are not in the area they're asking about even if they have decide to continue, I wouldn't get my hopes up). They have specifically requested to be contacted on the phone number they've included as well, to be fair.
If you want to respond to advertisers here who have not included contact information in their posts and would like a response you'll need to include some contact details as they are not permitted to send or reply to PMs. It's a good idea to set up a completely separate email address/SIM for this though, then if they turn out to be unscrupulous, annoying or just plain nutters you can bin it.