Well, the least you can do is be grateful they ask. They are at least concerned. A lot of buyers often complain about sellers selling them anything they want without doing too much research yet some buyers again are doing the reverse blaming the sellers for refusing a sale. Which exactly do we want? An intensive chat with the seller often do wonders. Let him/her understand your reasons for buying a fish and if he/she is not convinced, you can always offer backups for your claims that is, if you even have the patience to do it. I had an LFS owner impressed me by refusing a sale to someone who wanted some African cichlids with his angelfish. I was a new customer at that time. Naturally, he asked me as well before making a sale. I explained to him everything about my tanks and even to the point of quarantining them, he answered in response that people who had the patience to chat with the sellers deserve better than those who simply snub their advice and shrug off the feeling as they buy their merchandise. Just my experience.