Well, I can't really share a ton of experience with FBA...the linked blog earlier in the thread is really the best option for that. (I've been reading through the posts and not working on the project that I'm supposed to be working on, but on the bright side, I've gotten a lot of really good ideas!)
I use a Dell Axim x51 pocket PC and a SocketScan scanner. I picked them up on eBay for something like $200 (which included replacing the original pocket PC when it broke). As for an offline app, I like ASellerTool - the learning curve is low and their customer support is really good. I had to quit my account when I had two seizures and got my driver's license suspended for six months, so I couldn't go out to scan, and all they said was, "Wow, we hope you feel better soon. Let us know when and if you want to come back."
When I go out hunting, I download the database to the pocket PC the morning that I leave, so I have the most updated information. I figure as long as I make more than $30 in a month, it pays for itself, and as a bonus, I also pick up any new awesome books that I might want to read.
I don't pursue book sales as seriously as the FBA guy or some other FBA sellers...mostly I just enjoy looking through piles of books and getting paid for doing so is just another bonus for me. I do plan to pick it back up when I have more time to devote to it and can get around.
About the only real advice I have is that you never know what people are going to want to buy. I can tell you stories about selling a calculus textbook in twelve hours for $60, or how a book on internal combustion engines went for $125, but for every one of those, there are twenty more books that I bought for $1 and sold for between $5 and $10. Scan everything. If it doesn't have a bar code, put in its ISBN. Look for books in the following categories:
- non-fiction trade paperbacks (science writing, adventure, the occasional biography)
- religion
- anything on practical subjects (hunting, fishing, playing sports, home brewing)
- anything self-published or from a small press (mostly I just love these because some of them are fantastically messed up)
Avoid:
- hardback fiction (99% of it sells terribly; the only exceptions would be new hot books by fancy authors, I did a brisk trade in copies of Wolf Hall when it first came out)
- mass-market fiction (the little books that usually have barcodes on their inside front covers; the only exception is pretty much anything by Terry Pratchett)
- anything romance
- old textbooks (though it depends on the subject; old life science isn't usually worth it, but I had a big score last year of old and esoteric physics books, one of which sold for $75)
It takes some time, but you eventually start to get a feel for what's good and what isn't. Or what's likely to be good and what isn't.