Not really sketchy, people do this quite often. Though the profit you made doesn't happen too often in these situations
The reason I said it was sketchy is people are either A. starfishs or B. both starfishs and scammers.
Example A.
starfish: Can you buy me 50 rune scimitars? I can't buy any more. I'll pay you back.
Me: Okay, I bought them.
starfish: Jk I don't really need them.
Example B.
*Scammer puts key loop halves in the GE for 49,999 GP* (Actual price is 14,500)
Scammer: Hey can someone buy me some key loop halves? They're worth 50k in GE right now. I'll pay you back.
Me: Alright I'll buy them.
*I buy the key loop halves for 50k instead of 14.5k*
Me: I bought them for you.
*Scammer logs out with the cash*