I recently acquired a Polycom VVX 450 off Ebay for £30, which is an absolute bargain for a modern 12 Line SIP phone with OPUS / SILK support.
There was some risk however, it had previously been used by a VoIP provider that shares its name with a character in the Greek Alphabet and some providers lockdown the configuration to make it challenging to make them work with another provider.
Getting around these locks can occasionally involve intercepting traffic and redirecting it to your own TFTP or HTTP server, however with this phone it didn't seem to be the case.