ebay may work ok for the grab bag idea, but perhaps you can find out where the local comic book hotel shows are - and buy a table for $50 for the day. then try your grab bag idea. I did that ages ago (10+ yrs) when i cleared out my comic store. my grab bags were $1 a piece w/ two books per bag and then wrapped in brown paper bags or gift wrapping. Then I filled up long boxes full of all the grab bags and made a large sign that simply read "$1 each" - then I randomly inserted retailer premiums (golds, platinums, foils, ashcans, autographs, etc) to make it interesting. I did so well all the other dealers were bitching about how I was conducting a lottery - then they realized that I wasn't because everyone got 2 books for $1 and the cover prices were worth far more than $5. I had lots of premiums to go around - lots. My first show I sold out of 10 long boxes. Over the course of a few more shows I had sold most of my backstock, back issues, dupes etc. I must have had 500 Wildcats #1 (LOL) but I also had a huge stock of Spawn #1 & Shi #1 & #2, Rai #0, Gen13, Bone, McF Spideys, etc - that I used for premiums (back when they were rare and hot - loved the cover to Rai 0.)
At some shows I had people lining up and dropping every $ they brought to the show. Some people paid for my grab bags , didn't care about the crap books and gave them away or back to me.
It was so much fun I even went around to other comic stores buying they're overstock for 10 cents per book to use for my grab-bag fodder. I bought so many Valiants, Image, Dark Horse, etc. for .10 and flipped them the next weekend.
I dumped them at shows across the nation and made more at shows then my store.
sorry about the long post... just remembering good times... don't even get me started on how I dumped my Magic cards...
either way, good luck... even though I miss those days, I wouldn't want to have to inventory/transport all those books ever again.
have fun.