To be fair to the seller, I have had some ruddy nightmares with new buyers. You often have to hold their hand through the whole process, take a lot of time via email explaining how things work and they can take ages to reply and even longer to pay.
I personally am ok with this and accept bids from anyone, but a large volume seller may simply not have the time to deal with new buyers in this way. Don't be offended, someone else will happily take your money