"is it still available" is a perfectly valid initial question. For items that are somewhat in demand you will receive multiple offers,often quickly. It's not unusual for me to commit to a buyer, but while I'm waiting for payment I will not yet mark the item sold or pending. I will reply to anyone interested explaining what the situation is though.
If a seller is offended by inquiries maybe he/she is too temperamental to be a trading partner. I've been trading here and on ebay for several years now and have never been overwhelmed by "tire kickers". It's usually pretty easy to see how much time you need to put into a reply, and how long does it take to reply to a simple email like "is it available?". I agree with 2chnlben and mofi.
If a seller is offended by inquiries maybe he/she is too temperamental to be a trading partner. I've been trading here and on ebay for several years now and have never been overwhelmed by "tire kickers". It's usually pretty easy to see how much time you need to put into a reply, and how long does it take to reply to a simple email like "is it available?". I agree with 2chnlben and mofi.