Mile High Comics lists Detective Comics 168 at $770 US for Fair condition. My personal feeling on Mile High's prices is that they are "mile high" in most cases.
In 2009, a Fair copy sold for $262 US on Heritage Auctions.
Also on Heritage, a Very Fine copy graded by CGC sold for $4780 US in May 2011. That could mean a CGC graded Fair book would fetch about $560 US.
Doing a search on eBay doesn't turn-up a copy of Detective Comics 168 at all, so it's obviously fairly rare.
If you're selling it on eBay as an auction-style listing, expect to get as low as 50% of its current value. If it were me, I would expect to get about $130, and anything above that would be a bonus.
If I were selling it, I wouldn't bother with listing the auction with a Reserve Price (an additional $2.00 US or 1% of Reserve Price if that is over $200), but if you feel you have to get at least "X-dollars" for it, use the Reserve option. My experience in the past with selling rare items that command a certain dollar value is that I always have gotten more than the Reserve I set. Seeing as how this book is rare enough that nobody has a copy in any grade currently listed on eBay, it's safe to assume you'll get what you want unless you are expecting full price guide value. That's not to say you won't get more than guide, just don't expect it to happen.
I would suggest that if you want to get the most you can for it, list the auction for 10 days to get the most amount of exposure for it. This allows time for "word of mouth" to spread across various collectors boards, as the book is rare enough that there probably people always looking for it. If you're willing to ship the book to North America, I also strongly suggest listing it so that it ends on a Saturday or Sunday in the evening hours (as late as 11 pm on the East Coast) to get as many people bidding on it as possible.
Good luck!
