As someone who had to sell the line a few years ago, but recently got back in, the more popular characters seem to hold their value fine, and some even went up.
Some of the less popular characters (POP and NA in my experience, sorry fans!) are cheaper and often below original cost + shipping, but they're also not worthless (i.e. might go for $15 now instead of original $23).
Complete collections are always a tough sell, because it's a lot to spend at once, and many collectors will already have some of those pieces and don't want dupes.