Where were you thinking of selling it?
If you sell it privately or on a site such as craigslist, you will get a bit more for it since it's hacked, and if you find someone clueless about CFW you might get $50 more for it, or even more if you're lucky, but no one on this site would give you that much extra just because it's hacked, and eBay disallows hacked consoles, flashcarts and the like, and actively removes the listings.
I think $20 extra is worth it to most people for the convenience of getting the console pre-hacked, even if they already know about CFW. A lot of people just aren't willing to go through the trouble and risk (however small) with installing it themselves and would rather pay extra to have someone else do it for them.
If that's your only 3DS though, I would suggest you to hack it and download some games and try them out, you may find that you want to keep it after all, as I'm sure there are a lot of great games you haven't tried yet.