It may just be that like you mentioned, some people don't spend as much time on the computer as you or I do. However, once they're in contact with somebody about a potential transaction, I do think it's rude not to check their e-mail and follow up.

I e-mailed a company in a nearby city about doing some custom metal work a few weeks ago. My e-mail described what I wanted and had a link to an online photo of what the finished product should look like. I got a response the next day saying they could make the item for me and asking about some of the details. I e-mailed them back and asked for an estimate of what it would cost. Never heard from them again. I've sent one follow-up e-mail, but if I don't hear from them they won't get my business. Their loss.

For companies (and collectors) like this, it might be better if they never used the internet at all, instead of just using it when they remember to.