Money cheat.

Looks like they patched the money cheat yesterday, you cannot re-sell the cars anymore and if you try you will get flagged as a cheater. (Happened to my friend.)

Any idea on when there will be another money-generating tool?

There was this trainer that basically gave you 2k per second but seems like they patched that to. Tried it just now. Thread: GTAV Online Trainer By Felix The Furry v1.3.5.

And alot of other trainers and glitches etc have been patched or outdated, so just wait a little and it will propaly come out som sort of money cheat etc.