The same seller listing identical items via different accounts is actually a violation under eBay's duplicate listing policy, but I don't see much evidence of that being upheld; and funnily enough eBay actively encourage using multiple accounts in order to avoid fees thanks to the 30 free listings available to non-power seller accounts. 

Personally, I don't feel this particular practice is of great concern, except perhaps if someone is swamping categories with multiple (and therefore ultimately redundant) listings, i.e. hogging the search results, and as mentioned if you feel strongly enough about it to report, I would want to have no doubts at all it's the same seller (some people do copy listings, right down to the description word for word).