It took me awhile to be able to buy anything from Saibara too, and yes giving him good
clay and shipping it is very important in the process, but theres events that lead to the
actully activity of the shop being open....

::Note these are Magical Notes also::

Feat of Clay: Once 2+ hearts with saibara, and 5+ good clays, go to his shop....

Feat of Clay 2: 5+ w/ saibara and shipped 10+ good clays, go to his shop

Also heres a few hints to help you befriend him quicker
B-day: 2nd winter, give him stuff he likes on his b-day gain hearts easier and quicker

Stuff he likes: Good clay ^^ eggplant,fish,turnips,green herbs, and pickled food