They are surprised and disappointed. They may be afraid that they are going to make a mistake, or that you will take advantage of them. Customers who lack social skills may lash out at you. Handled properly these folks can become your best customers. If your products didn't satisfy at least some of their needs, they wouldn't expend the effort to object.
Some folks lead with a confrontational attitude.
Try not to get flustered. If you can uncover their real needs and satisfy them in a professional manner you just might tame a nasty customer and reap the rewards of repeat business and customer referrals. If you just can't deal with a particular customer, try asking the customer if he or she would feel more comfortable working with your associate or your manager. His sales meetings and seminars are presented nationwide to major companies and industries.