The reference number won't give you any tracking details on standard Large Letter postage - Large Letter items are only scanned at the point of delivery and then only if they have a Horizon label/barcode, there won't be any tracking of any kind if the LL is posted using stamps.  Standard 1st and 2nd LL are not scanned elsewhere in the network (and often don't seem to get scanned at the point of delivery by the postie either, it seems). 

Only an actual tracked service will give you any tracking details during transit. 

Unfortunately, since the changes to the requirement for delivery confirmation, ebay will now hold on to your money until after there is either conformation of delivery, or feedback left by the buyer.  If neither of these happen,  it is longer for the hold to expire.