Chat channels dedicated to HyperNet are probably the best way to advertise your offers right now because of the constant spam fest.
By blocking the spammers you could miss out on some great deals.
While we wait for CCP to maybe shorten the HyperNet chat links or/and add more ways to filter the search, here are some in-game channels:
“Hypernet Hub” (probably the most popular one)
“HyperNet Relay” (not as popular but still good)
“HyperRelay Trading Public” (not really that popular either but people still post offers every now and then)
“Tishy’s HyperNet Network” (slowly but steadily gaining more and more people)
“KMKHyperNetDeals” (shameless self-plug, links to offers in MOTD)
If i missed any or you have your own channel feel free to comment on this post.