Disclaimer, I am a US Citizen, but this question's sat for a bit so I'll offer you insight as a Public Policy PhD.
Generally speaking, when it comes 'store credit' which is what this Steam money basically is, the reason the store gave you credit matters. If you received the store credit because you returned merchandise then no, that is not income for the purposes of taxation because it's not being used as payment to settle a debt - it's being used as an intermediate step in the remediation of a contract dispute.
If, however, the store issued you such credit in payment for a service you provided then yes, that is taxable income. It becomes taxable income in order for tax law to have any meaning, and it is 'income tax' because it is a flow of value from a buyer of your product (service) to you, as a vendor. Otherwise, your employer could avoid paying taxes by granting you "store credit" in exchange for your labor, instead of money.
Even though you cannot turn them into 'grocery money' there is a concept in laws pertaining to money: fungibility. A resources is fungible if any given instance of that resource can be used in place of any other given instance of that resource. Example: Any 5 Euro note can be used in place of any other 5 Euro note - Euros are fungible to each other. A 5 Euro available credit balance on a credit card can be used in place of a 5 Euro note in most cases - available credit, in Euros, is fungible to Euros.
A 5 Euro balance on a Steam account is fungible to Euros if you buy video games on Steam valued at or more than 5 Euros.
Basically, the argument in a court of law would be that since you likely would have bought video games anyway, the 5 Euros of Steam credit would have been covered by 5 of the Euros you spent on groceries, and therefore it is equivalent.
The bottom line is that yes, you are selling things on the internet, people are paying you for those things, and you are receiving something fungible to cash. This is basically the textbook definition of income, congratulations on your small business!
HOWEVER:
You are probably not making enough money by this route to break out of a likely 'hobby income' provision in the tax laws. I am not a tax professional, let alone a Belgian tax professional, this is not legal advice, but any sensible system of income tax is going to have escape clauses for kids running lemonade stands, or knitters selling the occasional scarf to a friend. In the United States, income from a hobby is taxable, but as personal income - you needn't file business taxes over it.
You are probably not worth auditing. Audits are expensive, so the trouble you're likely to get in for not - or mis-reporting this income is very, very small. A government's tax agency is a rational economic actor. They will only audit you if they think there is a reasonable chance of discovering that you owe more in taxes than it will cost them to perform the audit. Your Steam trading card farming would have to be pretty insanely lucrative indeed. Don't kill yourself trying to keep books on this, just glance at your Steam balance history, make a good faith effort to arrive at a reasonable estimate, and report it as some miscellaneous income category (in the US, it'd be 'hobby income.').
