1) Has anyone here either heard of or done business with Tripsta?
No.
I take it you have googled and read some online reviews on places like Trustpilot or TripAdvisor?
Make sure he uses a credit card so he gets the benefit of 75 of the Consumer Credit Act 1974 if the transaction value is more than £100. Just to re-iterate, that's credit card
NOT debit, or Amex or Paypal or .... !
The checkacode site shows the IATA numbers to belong to agencies in Greece.
Looking at the T&Cs on the Tripsta site, Tripsta is actually
Travelplanet24 S.A t/a Tripsta and they publish a PO Box in Athens as their address and refer to Greek law in their T&Cs.
2) The Finnair flight is operated by FlyBe is this a simple charter arrangement or something else? (FlyBe do operate some domestic Finnish flights).
Codeshare maybe ? FlyBe and Finnair did have a joint-venture until recently.
3) Given that it seems to be a "real" Finnair flight I'm guessing the baggage is the Finnair allowance - 1 bag?
Depends whose metal he's ultimately flying on, since that will decide the allowance.
Oh, he's got some BA Avios so I've sent him off to see if he can get a part cash/part Avios deal.
That would be my first choice.