How do I deal with items I'm selling for others? We agreed on 60-40 split. If I sell a $10 item, do I enter $6 as an expense and $4 as income? is it that easy? Thank you

You enter $10 as income and $6 as an expense. On your tax return the expense gets subtracted from the income, so your net income will be $4.

if you are a consignee - In other words, you don't own the item all you enter is $4 your commission for selling.  if you do own the item you enter $10 for the selling price and $6 for the cost ending up with a  $4 profit.  if you enter $6 as an expense and $4 as the income you end up with  a $2 loss which is totally wrong. 

Since you will probably get a 1099 reporting the gross income your business takes in so  I highly recommend you report 100% of the gross income coming into the business (so it matches or exceeds what is reported and avoids an IRS CP2000 notice later)  and expensing what you pay out to the owner of the item.  You may also want to issue them a 1099 to support this deduction.