This can be sold "legally" for the site? I havent find also no statement on this...What did you think? How much you will value with 0.03-0.06 bonus btc inside?
In my opinion, it's worth nothing.
If you claim a bonus, the same amount of bitcoin will be deducted from your balance and it goes to your bonus account.
For example, if you claim a bonus of 0.03 BTC, another 0.03 BTC will be deducted from your balance and you will have 0.06 BTC in your bonus account with the wagering requirement of 1.5 BTC.
Given the high house edge at freebitco, you will probably lose all your bonus account balance before fulfilling the wagering requirement.