The very first section of the Arena page is very clearly marked as a contraction area, not a building area.There is a separate area on the page to build seats.
I would dispute the "very clearly marked" portion of your statement. Also I understand that others have had this problem as well, so I don't think it's very clear.
Sell Seats from Your ArenaDuring Season 9, we implemented a new system for allocating TV revenue that was more realistic. Those leagues in higher divisions or with the most superstars are now able to broadcast their games to a wider audience. While weekly TV revenue for these teams has increased as more people now follow their teams from home, in Season 10 it will be much harder to fill a very large arena, and some teams may be left with many empty seats.During Season 10 (and only during Season 10), the BuzzerBeater commissioner is allowing all teams to "sell" some seats from their arenas to account for the new attendances. It will take approximately as long to sell a group of seats as it did to build them, and they can be sold for the same construction price. You will receive the money for the sale only after the contraction is complete