If a company takes money from your sale, for example 30% in some cases, could you claim that as a loss on the sale? I'd like to use a medium to sell a product but 30% is crazy!
If by "loss on the sale" you mean "an expense", then yes that is fine.
Say you sell a $20 game on Steam (30% is very common on markets like Steam, Google Play Store, iTunes Store, etc...), then you would report:
- $20 as Income
- $6 as Expenses
Side note: 30% is actually not that bad. For a product I helped bring into a few retail store chains, they are effectively taking closer to 40%, though retail can vary depending on several factors.