If you compare the amount of numismatic and sigillographic material that is published online, you'll find that only a relatively small part
of seals is available online. This may change over the next decades, but right now, that's simply the case. Only the collection
of DO is available online, but not yet completely (but I think the seals published in their books are all there). Another great resource is 'Prosopography of the Byzantine World' which lists a large number of seals, but only readings, no photos. All the other online resources are so fragmentary that they are not very useful. So it really depends on how you consider yourself as a researcher/collector. I think Zacos
I and II are pretty much a 'must buy' to every collector with Byzantine seals
as main subject. Simply because they describe an enormous amount of material (3400 examples, not counting multiples and quoted specimens) a substantive part
of which can't be found elsewhere.