I would correlate that with real life.
I feel that investing money in anything, be it 'pretend' or not it can't be absolutely sure because otherwise it's an infinite source of money.
Although there are some investiments that have a very low chance of 'going awry' and therefore are considered good investiments.
I see that if you mean to invest, the only 'hard work' you mean to do should not be something such as 'buy supplies with it and go hunt', that's investing but I don't think that's what you mean, it should be at the Auction House / Market, buying and selling and that's where I'll begin.
I have done this on other MMORPGs and I've been extremely successful in all of them, except Tibia where I did just 'OK', nontheless, here's what I did and do.
- Imbuement Items / Rares / Potions / Items in general
I don't do those, they're directly related to the amount of players, monsters they hunt and so on, that makes the market wide and too volatile.
After carefully watching the market for around one month you can evaluate the 'average' price and have an idea of when most people buy and sell and for what value they can go and most importantly, when that happens.
With that in mind, I buy TCs and Resell them at the best time.
Take in consideration
IRL payment - from 25th to 10th day of the month - People earn their money and then spend it.
IRL timezone - If the server population is predominantly from South America, then most players might be on from 18 to 2am (usually that's when most leave work/school in South America)
Tibia events - Rapid Respawn - Devovorga - Morgaroth - and so no
Player organized Tibia events - Quest services, Forum gatherings and so on.
So, with careful planning and some critical thinking, you can get a nice excel sheet and do some business inteligence and properly make some money.
I do recommend some further reading in other aspects, such as, doctorate and master thesis about Macroeconomics on the Digital World, about the impact of cheating on the Tibian market and so on.
At your own rist, you may take a look at a Thesis from Stephan himself about Tibia.
Let me know if you'd like to know more about it, I can elaborate.