Is selling something at a large profit ona'ah (wronging another)?

Question

What would sources say about the following scenario: let’s say you buy a pen for about $20 and two erasers for $5 each, equaling $30 in total. The person who bought these things sells them for $40, making a $10 profit.

Would sources consider this onaah?

What if I would sell the item for $50?

Answer 1

You are referring to the laws of ona'ah (unfair pricing) based on the gemara in Baba Batra 90a

And one who profits from his sales may not profit by more than one-sixth.

However,

• the reference price for the calculation is the fair market price, in some cases this can be regulated (e.g., for basic food staples), otherwise the price which applies is "any price that the majority of sellers would consider a marketable price" (see here, #2)
• a retailer who sells piece by piece is compensated for his effort and all his expenses, and may profit a further sixth above this sum (from here, B4, based on Shulchan Aruch CM 231:20)
• it is also possible the law only applies to essential items

So a markup of 33% is completely fine in regard to these laws, as fair compensation for the expenses involved. Indeed a 2x markup is considered standard in many areas of retail as compensation for rental of sales space, salespeople and other costs.