You will then have the option of paying «online» via credit / debit card or «offline» by cheque, transfer or cash NB exactly which options are available are dependent on the Artist, e.g. some artists may not
accept transfers etc..
Aldiko and Kobo both
accept epub, pdf, and Adobe ACSM so you would be able to
transfer all of your existing books but I would expect there would be no way to
transfer setting, bookmarks, highlights, annotations
etc..