Personally, I'd break them down into a couple of smaller lots. Manga can go pretty cheap (I can often get new volumes for $2 from a store near me), and just to send one volume, the minimum postage cost would be $6.95, so selling them off singularly wouldn't be of much benefit to you or buyers.
If you break them down into series (if applicable), or just lots of say 3, 5 or 10 (in chronological order), you can reduce the average P&H per book, still offer the opportunity for combined post discounts, and not put the start (or end) price out of reach for someone only looking for a volume or two.
Lot size would probably depend on getting the best value out of a single P&H charge (eg take a look at how many volumes could fit in a C&S flat rate 500gm or 3kg satchel, or chec kout the C&S flat rate boxes).
