Has anyone ever bought from Audio Liquidators

I believe they are a California based company. Their website says they have been in business since 1981
In business since 1981 doing what? Caution, especially with a credit card transaction is a must. Be prepared to  cancel your card quickly if any difficulty with your transaction occurs. Never use a debit card with questionable vendors.  Force a transaction into PayPal if possible, for they will provide some limited protection, mainly if goods are not delivered, not for issues with the goods received. Years ago, one of the major banks had a feature in their credit card product that allowed you to generate a new card number and limit the amount of the transaction, as protection from scams.  Sadly, they dropped the product. Remember, if the prices are too good to be true, you are being conned.
After looking at they're web site I saw that they had a warehouse location close to me. I called them to see if I could pick up a sub woofer instead of delivery. They never returned my call. I'm certain it's a scam.
You should read the company's warranty policy thoroughly. Since they are not authorized dealers for what they sell, warranty on any products they sell would be handled through them to a third party servicer. So in addition to the other risks mentioned above, there is no manufacturer's warranty with their products.
Never would buy from these guys.
I was looking at their subs and I see they are selling the REL S-3 for under 1600.00. This sub retails for 2000.00. If they are not an authorized dealer how can they buy it cheap enough to be able to offer it at this price?

Call the police.  Seriously.  I've done that when I have a street address and asked them to pay them a visit.  The also can tell you if any police reports have been filed about them.  Yes, most police departments have better things to do, but on two occasions it worked.  One turned out to be okay, the other not, being a non existent address.
Check Music Direct, today they have the S-3 for $1500, and they are an authorized dealer.

Is that a new sub or a used one? I have seen used one's on Audiogon for 1500.00.

I took one look at their site and smelled a rat.  

Something fishy.  

Shop elsewhere.