I think we are safe to assume the OP isn't talking about sellers in the TP classifieds having only been a TP member for 3 days and 9 posts.
It sounds a little unlucky to have 3 sellers not reply but there can sometimes be genuine reasons for no replies. It's possible the seller may not have been on the site or may not have got the notifications that there is a message. Not knowing what sites you are referring to its difficult for anyone to say if thats unusual or not. What I would advise when buying your first camera and looking at buying used is to buy from a reputable used seller like
MPB, WEX
LCE, or similar well known company/shop. The reason I say that is because you will know its been tested and you will get a warranty too if you later fine a problem (my first nikon had a memory card issue and
LCE repaired it under warranty). Even if you go and look at cameras will know what to look for and how to test a used camera properly? TP is a great place to buy gear from as sellers are genuine and there are very few incidents of problems. I seem to remember access to the classifieds is limited by a minimum of 60 days and 25 posts but I may be wrong.