This isnt any intense secret or anything, but its an interesting concept:
I used my sisters DS as well as mine.
I used hers and went for many walks using the park save cheat and tried to get as many expensive items.
Then I did bark mode, and got her dog to bring me all of the expensive gifts it found by doing bark mode repeatedly (you can only bring one item at a time)
then i sold those items at the second hand store and got money for them.
like i said, this is nothing top secret but its great if you have 2 DS's and 2 nintendogs in your family.