Totally agree they are red flags, and I make it a rule to personally never do any business with any company on the web that don't provide me with at least two forms of contact, and at least one of those has to be offline communication.
When you deal with any company that only lists an email address for contact. The biggest issue is that it makes it far too easy for them to simply ignore any complaint they don't want to deal with and that leaves you up the tree with no way down at all.
IM is the same, don't be fooled into trusting a company because that provide you with an MSN address or some other. Only takes 2 seconds to block an ID and again, you're back in the same tree, with the same problem.
I always ask for a phone number and if non is forth coming, then they don't get my money!
Having said all of that, the liquidation market can be a little different. If the company is selling direct to the public as their primary market, then they will usually provide you with all sorts of contact information, because it's in their best interest to create sales.
If the liquidator is selling more b2b, they may well hold back their address and phone number, the reason being is because the may well only have limited staff, and the last thing they want to do is have their time wasted by people ringing, or turning up at their warehouse asking questions all day long.
Those of you that have run a business know how much time can be lost in a a day simply servicing inquiries that end up leading now where even close to a sale.
So as far as red flags go, yes, definitely something to be weary of, but may well be just fine. Easiest thing to do is contact them via email and ask if they do have a number that you could talk to a consultant on.
If they are genuine, they will see you as a serious inquiry and will most likely give you a number for contact purposes. If they refuse, then I would be asking the reasons why, and then basing my decision to move forward with a purchase only after exhausting my avenues of research.