I recently sold my N900 on eBay, the phone was a refurb from Nokia because my original's charger port broke, it was completely new from the factory apart from my old shell on the back as far as I could tell.
Anyway it was sat in my room for two years just collecting dust as I jumped ship to android, I cracked it open last week, gave it a charge, tested it and stuck it on eBay.

The person I sold it to sent me a message saying the phone charges intermittently, maemo browser often freezes and he took it to a phone center and they say it was overclocked.

I have a feeling the bloke is trying to pull a fast one and swapping his broke one for my new one as when it left it was absolutely perfect (well, I don't know about maemo browser). Is there anyway to prove the phone is the one I sent?

Only if you made a note of the IMEI number.
Or if he hasn't reflashed it may have some of your old data.

It's best to post on here for genuine buyers, ebay will cause you grief.