I have a little aromatherapy kit that comes with a book about aromatherapy, 7 or 8 little incense cones and two little vials of scented oil.... can I trade that here? Can I send it in the mail if it has liquids in it? It's really not a very big kit, I've sent much larger books. What I'm wondering is does that classify as something I can send through this site? It does come with a book...

Sorry, you cannot swap it here. from the help section:

PBS is for the exchange of used books across all topics of interest. Paperback, hardcover and audio books in good condition are acceptable here. 

Guidelines below

Allowed:

• Paperback books
• Hardcover books (dust jackets not required)
• Audio books (must be originals, not burned copies)
• Ex-Library books
• Books with accompanying media
• Textbooks/Workbooks 
• Foreign-language books (if in a language other than English, the language must be specified in the book description)

Not allowed:

• Anything that is not a book
• Anything that contains advertising
• Advance Reader Copies (also known as ARC s, Review Copies or Uncorrected Proofs--these are not the final versions of books)
• CD-ROM books/e-books (books that must be read on a computer)
• Burned copies of audio books
• Xerox or otherwise unoriginal copies of books
• Unbound pages
• Leaflets or pamphlets or booklets* without ISBNs (other members complain at spending a credit for these)
• Books that require supplementary non-media materials (non-media materials cannot be sent by MediaMail)
• Pornography (sex manuals and erotica are allowed, hardcore pornography is not allowed)

*A leaflet/pamphlet is an item with fewer than 20 pages; a booklet is an item with between 20 and 50 pages.

Please note: if you post an inappropriate item for swapping, it may be removed from the database as an automatic action without notice; if it has been requested from you before that happens, you will not get credits for sending it.

Don't know if this is helpful, but it sounds like something that would be appreciated on makeupalley--my friend is there, and it's basically the same concept as Paperbackswap--just with cosmetics and such. http://makeupalley.com

You could offer the "extras" as a bonus in the Book Bazaar forum.  For postage, you would not be able use Media Rate.  As long as liquid items are well packaged its OK to send them via the mail.

There are plenty of perfume sellers on Ebay that ship using the post office - but they always use Priority Rate. And make sure to let the post office know so they will stamp the package for careful handling.

And no, you can't post it here because of the "non-media supplementary materials" rule.