I second the first part of what neotank said but the rest is just pointless. There is no reason to UNHACK a Wii and its more valuable as a hacked Wii. Just like a PsP with custom firmware is more valuable than a Psp with factory firmware. The only one that gives a damn if its hacked is Nintendo. You gain NOTHING from UNHACKING a Wii, at all. Unlike having a hacked 360, there is absolutely no down side to having a hacked Wii. The Wii never gets banned from the network, and retail games still play in the unit.
Keep it hacked and tell the person you are selling it to that its hacked and you will get more for it. Unless you are selling it back to Gamestop or something (which I dont recommend because Gamestop has horrible buy back prices). In that case dont tell them shit, they wont give you extra for it being hacked (usually they will only give you 1/4th of what they sell a used Wii for). Just if you are selling it back to Gamestop or a store like Gamestop, make sure if you have your Wii set up to boot to the Preloader to change that. Also it would be a good idea to move the homebrew channel to a different page other than the first page. Reset or delete any customized system settings, mail and/or save files (if you care about ppl playing your saved game files) and you are set.
Word of advise: Sell it back to a BUY SELL TRADE second hand store, trust me they give you alot more for your stuff.