Collectors of series books, bless their hearts, are a passionate and dedicated group. Anytime that I get a good series title in and list it in my inventory I can count on calls from collectors not just for the title listed, but for other titles in the series that I might have stashed away or may find in the future.
The problem is that I have over 13,000 children's books online, at any given time maybe 10 of them are series titles. I've found that serious collectors of series books are not usually interested in collecting other genres, and to specialize in series books as a dealer takes a major commitment in time, reference material, and most importantly space.
I never mind getting a call from collectors wanting to talk books, I always come away learning something, like the last book of most series is the most valuable etc. Kind of like the market research coming to me. But it does get frustrated at times to get calls that can't turn into sales.