Some more information would help.  What is the model name of the alto?  Maybe copy the link and post it so others can check it out.  Be careful, there are a lot of fake Selmers on line nowadays.  The comment about no serial number doesn't sound right.

That price sounds awfully low, so caution should be exercised.  I know that every young child wants a shiny new horn, but it is generally not a great value, expecially in student horns.  A new horn will depreciate about 50% almost immediately.  You would probably do better to find a good used horn like a Yamaha YAS 23.  They are well made, and if your child loses interest, they can often be resold with little or no loss.  But they aren't shiny new.  

Final comment, $450 shouldn't be less than 4 months rent.  You can find better rental deals than that!