On fixed price items, use the add to cart function instead of 'Buy It Now', as it shows you up front if the seller won't post to a PO box and won't let you purchase / complete checkout (unless you change the address).
I did get caught out once because the seller had specified Aus Post delivery services, and had not blocked PO boxes, so bought, paid and then received a courier tracking number. On re-checking the listing, I found right at the end of a long description, in a non-visible tabbed pane, that they were using couriers and couldn't post to PO boxes, so that was a bit annoying but got sorted fairly quickly as I can provide a street address when necessary.
I do not recommend using alternative ways to spell out "PO Box" to get around seller blocks on them, I see it all the time on buyer addresses I get (not an issue, because I do post to PO boxes), but I empathise with sellers who specify that they don't and get this, because one it's not their fault eBay will allow purchase when they don't send to PO boxes, and two (to me) it's basically saying "bugger you, you'll post the way I want you to".
