That seems unlikely, Nintendo stopped blocking DS flashcarts in 7.0. (i.e. if it worked in 7.1 it'll work in 11.0). You can install flashcardtimewarp.cia which will reactive are flashcarts that were blocked. You can use ApacheThunder's TWL mod to reactivate carts that were blocked on the DSi.
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You can actually follow the guide completely as written and get rid of your emuNAND (the guide will get you to flash it to sysNAND). Then, you'll be able to set it up so Gateway either boots automatically, or boots when you hold a button at start up. In my opinion this is the best way to go. Keeping emuNAND has many disadvantages. You will want to use the launcher from 3.7.1 for this (the 4.0b one has an annoying popup every time you boot up Gateway in sysNAND mode).
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Go to system settings, other settings, then system update.
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It depends what your buyer will pay really. If you're in a city with a big population you have a better chance at getting a good price. I would start off advertising it at double what a used console is worth, and reduce it every few days till you get a bite. If you include a link to the Plailect guide in your ad then people might be more impressed by it (otherwise they don't know if it takes ten minutes like the Wii or is actually a time-consuming job). Posters at your local shops are useful too because then you can attract some non-tech-savvy people that don't know how to browse online classifieds, or who weren't necessarily looking for a 3DS but wanted one when they saw your ad. Good luck.