It is usually the nature of economics that money is usually in exchange for time or skill. The stuff that people will want to buy will be the stuff that they either don't want to work towards, or can't get at their current point. The money-making scheme usually comes in putting in extra time/effort to achieve economies of scale, and produce in-demand things on a larger scale than would otherwise be viable.
As such, simple resources (uranium, wither stars, etc.) might be worthwhile (if you are able to set up a sufficiently large system to get them). Alternatively, you could do something like make a T5 pigman spawner, and upconvert all materials, selling the diamond blocks as effectively universal resources. I've found that copper, tin and silver are a pain for that kind of thing, too.
Alternatively, you can charge for processing. Possibly take in materials and a fee, and output a complicated item (high tier solar panels, gravisuits, etc.). With a decent autocrafting system out back it can make life easier.
Charge for energy (perhaps have a trading station where the input is a lapotron crystal/equivalent and money, and the instant output is a charged version from stock, while you system charges theirs).
You could charge for something which would otherwise require grinding, such as higher bee species. The pitfall to be aware of here would be that they are replicable, so there is a danger of "priracy".
Finally, you can charge for a service. Say the set up of a nuclear reactor, or similar.
