As a collector of old magazines - I've learned there are a few things you need to be aware of before purchasing.

1. A lot of the old bound magazines being sold today have been stripped of the most valuable prints.  This is especially true with Scribner's and Century Magazines from 1890 to 1920.  A lot of the Parrish, Wyeth and Christy prints among others are being removed before the magazine is listed.  When this happens the dealer can sell the prints separate from the bound volume.  Normally, this is not a problem, unless you are using a collector index, and over bid because you think those prints are included.
2. Condition is often mis-stated.  Bad condition is often covered up with words like "good condition for its age."  The fact is - Just because a book is 100 or 150 years old, does not make it bad.  Condition should be stated in terms of the binding.  Is it tight, loose, or pulling apart?  Are the covers attached or at least present?  Are there loose pages?  Are all of the illustrations called for present?
3. Another thing you need to look for is foxing and spotting.  Is it heavy, moderate, or very light.  Be sure to look at pictures to ensure your judgement meshes with the sellers description.

Pictures tell a thousand words.  Be sure to look at the pictures included.  If defects are mentioned in the description - does the seller have pictures to illustrate the problem.  If you are just looking for a reading copy this is not all that important, but if you are putting together a collection - Pin the seller down.  Ask for pictures of any problem areas, and agree in advance on return privleges if the item is not as you expected.

I have purchased thousands of bound magazines and books on ebay, and the one thing I can tell you is - That one of a kind item will come around next week, or next month - so if you are not sure on the item...Let someone else bid on it.  Keep your eyes peeled for the next copy.