Can anyone please advise me as to whether it is a track or a triathlon bike?
Looking at the hub spacing on the rear and the bb height it seems to be a track bike. The seller says it is a tri bike.
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Looking at the Argon 18 website it is almost certainly a triathlon bike. The other E series bikes are listed in the triathlon/time trial section, and their track bikes use a different naming scheme. The reviews I found also call it a triathlon bike. Then there are the brakes and derailers (and triathlon bars) mentioned by @ojs.
Given the difference between track and triathlon riding and the rather obvious differences between bikes meant for each discipline (derailers vs. no derailers for starters) it seems quite odd that the seller can't tell you what type of bike it is. I'd second @RoboKaren's suggestion that you check it out as a possible stolen bike.