Because the Harry Potter books have been mass produced they are not actually very valuable for resale. The true first edition of the first US book is one of the few valuable US Harry Potter books. The audio books do pretty well for resale, I have never seen one of those at a thrift. All the Harry Potter books say first edition for some strange reason even if they aren't a first edition so don't go by that. Nice to find a set in mint condition though if you are a fan.
My daughter is a huge HP fan but unfortunately I haven't found too many HP collectibles at thrifts. Nowadays any little Harry Potter thing gets overpriced anyway. The few things I have found are a Hallmark HP mug that is apparently HTF and goes for 30-40 bucks or more on Ebay, I got mine for less than $1.00. NIP HP viewmaster set from the first movie. Mattel Hagrids Hut playset and Hagrid's gift figure. I have also purchased other figures and stuff like greeting cards, stationary etc. when they used to be priced less than a dollar, but now when I see stuff like that it is 5 dollars or something ridiculous.
We went to the Harry Potter exibit at the Boston Museum of Science back in Jan. it is a traveling exibit that was going around the U.S & Canada. If it stops at a museum by you I recommend it, it has the costumes & set items from the actual movies, not reproductions.
If you can catch a Harry & the Potters show they are a fun band. They are college students, but do tour pretty often.
http://www.myspace.com/harryandthepotters