Hi! I’m new here, but I wanted to contribute my easy pre-Sonata City money method. I found this thread by researching IV Stone farming methods. I think some folks might find this useful, maybe…?
Requirements: Old Rod, given by a Fisherman in Route 2.
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When you reach Crossroads, catch a Kecleon. It will know Thief. If it doesn’t, catch a Ditto too, breed a Kecleon, and train him up until level 25 or something, or use a Heart Scale. Mightyena is also an option, but it doesn’t learn Thief until level 57 so you might as well go the DexNav method.
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Go to Suntouched Exitway. Start fishing with the Old Rod.
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Use Thief on every Shellder that bites, run away right after. Shellder rate is 70%, so you’ll see them frequently. 50% he’ll be holding a Pearl, which sells for $700, and occasionally a Big Pearl, which goes for $3200. A message will pop up when you actually stole something. Remove Pearl from Kecleon.
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When you run out of PP go back to the Pokemon Center to heal and sell your Pearls.
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Repeat 1-4 until necessary. EDIT: Some quick math. I’d usually get somewhere in between 12 to 16 Pearls and 1 to 3 Big Pearls per fishing trip, which is consistent with the 50% and 70% rates mentioned above. Thief’s default PP is 25. This nets you an average of 13000 coins or so for every 32 to 35 wild encounters.
This was enough to tide me over until I got to Sonata City 
I wouldn’t spend too much time
Pearl fishing, just enough to get your Secret Base set with the minimum and the backpack stocked with essentials.
Kecleon can be used to farm Heart Scales from Luvdisc as well. Earliest Luvdisc spot is Midna Cave with the Old Rod, 20% rate. Most frequent Luvdisc rate is Nexa Border with Super Rod, 40%. Luvdisc have a 50% chance of carrying a Heart Scale.
Making the same math as before: Midna Cave spot rate is approx. 12-17 Heart Scales for every 125 wild encounters. Nexa Border rate is approx. 12-17 Heart Scales for every 65 wild encounters.
Also EDIT, two side notes:
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Devon Corp as a Sonata Races sponsor gives out a LOT of Pokeballs: Most of the time I get an Ultra Ball, a Dusk Ball, and a Timer or Quick Ball or Great Ball alongside my money. Considering these balls are usually like $1000 a piece, and you can never have enough of them, you gotta factor the money you’ll save in your winnings from the races. The tiny tiny chance of getting a Master Ball is also tantalizing.
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Rock Smashing is not the most efficient way to farm Heart Scales, BUT if you’re trying to obtain IV Stones, you’ll get some too. I got about 25 Heart Scales to my 8 IV Stones (so far). So I’d wait until you got your fill of IV Stones to go Luvdisc fishing, if you were planning to do both.