I respectfully disagree with you there. If you have ~5000 or even ~10000 Robux, it is impossible to make a significant amount. The odds are that you'll be making only 1-3 Robux in profit during each trade (meanwhile, you're spending almost forty minutes doing this). Also you've just been extremely lucky, it is extremely rare to double your entire capital within a week and a half with the TC (with the amount you have). If you had 400k or 500k, I could understand. 15k though? That's luck, and good on you for getting the good rates.
I hope you were exaggerating with the numbers of profit, because when I calculate even 10000 R$ I get this :
Going with the worse / very common RATE profit trade that can happen
10000 R$ x 17.15 (ROBUX RATE) = 171500
171500 TIX / 17.14 (TIX RATE) = 10005.8 R$
Alright so nearly 6 R$
Most trades for the last month have been the following rate gaps (0.05 - 0.1)
So let's go with the same number (10000 R$) with 0.05 rate gap :
10000 R$ x 17.15 (ROBUX RATE) = 171500
171500 TIX / 17.1 (TIX RATE) = 10029.2 R$
Now with 0.2 (least common, but happens daily):
10000 R$ x 17.15 (ROBUX RATE) = 171500
171500 TIX / 17.0 (TIX RATE) = 10088.2 R$
I mean I get what you're saying, it's chump change right?
However, you're forgetting that people can cycle their R$ / TIX on Trade Currency 3-6 times a day.
I peaked 2 days ago at cycling 11 times.
It adds up, your "interest rate" will be high. There is no luck involved just predicting the highs and lows, especially with the bots on it right now.
I only don't do limit trades, I do market trades when bots try to be aggressive towards me, making me a lot more R$ in the process.

Also trades take me 10-15 minutes, I don't know where you are getting 40 minutes from unless you're aiming too high. I only check my trades once every 1 or 2 hours.
Since I don't have that much time on my hands it's just fun playing with this, even if developers place 10M TIX and sweep everyone's R$ including mine out of nowhere, gives me a challenge.