Some of my best ever punts were from vivastreet. However like mentioned it's been ruined by phone scams etc. It's a massive shame as I've found real gems on there in the past. A few tips of you want to use it..
Make sure the pictures are believable and not heavily edited. Don't ring any 070 numbers they cost you. Have a look at the username of who posted the ad. That can give alot away.
Have a decent chat on the phone and ensure you clarify everything.
If you get there and it don't look or feel right just be ready to walk and not look back
Hope this helps