I had the same thought. I gave up on tipping after I got to the 3rd level and became Arte's "friend" by buying things on sale.
For example:
The Ballet Coffee Table sell in the W shop for 300KC
If Arte has it on sale for 30% off, I wold buy it from him for 210KC.
Then, (since I already had one) I could sell it back to the W Shop for 150KC.
I got credit for 210KC with Arte towards becoming a friend, but really was only down 60KC (210-150)
...Does that make sense?

Obviously, you get the biggest bang for your buck by getting the items that are 30% and 40% off
