Comics are one of those things that everybody thinks they "know the value of", and damn near all of them are wrong. It takes months, if not years, of hands-on experience and continuing education to be able to reach a point where you're semi-competent in understanding the difference between "what a book is worth" and "what some guy listed it for on eBay".

The store where I work buys and sells comics, but there are only a few of us on staff who I'd consider to be truly qualified to quickly assess a long box and determine if there was anything of value. The "how much can you give me for..." stuff we head off over the phone in much the same way as you do, since it's store policy to never make offers without seeing the merchandise first. A customer's definition of "good condition" and the store's definition of same are often wildly different. :)

Fortunately, most people selling to us have already been to a full-fledged comic store, and anything with any major value has been sighted and pulled already. That means they know what's left is essentially worth very little except for bulk sales, and they rarely quibble about the price we give, since the alternative is for them to take them all back and get nothing.

Every so often though, we get the Johnny-Come-Lately who looked all his books up in a value guide and "knows what they're worth". If I deal with these types, and they throw a fit, I calmly remind them that to get the best value, they'll need to list them online individually and wait for them to sell. Watching them do the mental math for the time it would take to photograph, write up, and list five hundred individual issues, not to mention how long it can take for comics of low value to move, they usually come to the conclusion that it's a better use of their time to simply sell them to us for 5-7 cents apiece, take the money, and run. :)