I use both the lily auction and facebook. Both have pro's and con's. I have sold lots of seeds on the lilyauction and some plants. With the lilyauction you can get more bids and sometimes sell a product for more than you expected. Facebook the product sells for the listed price.
Everything is free on facebook , so anyone can create a page or a group. Facebook seem to attract a-lot of new people so you never know if you will get paid. On the lilyauction Mike Is there to back a seller up. There are also fees associated with the lilyauction .
I only buy from a small number of people on the lilyauction and have gotten some excellent deals. I usually don't buy much off of facebook unless I know who I am dealing with.
Facebook is also a good way to promote your business because you can create as many pages as you want for free. When new intro's come out they usually appear on facebook first and some are sold out even before the general public has seen the plant.