HP15C displays gibberish. Is it broken?

[rprosperi]

Valentin and Max's suggestions to ask the seller to do the test and/or remove batteries make sense IF the seller will do them; the results will remove most doubts.

Otherwise, I agree with Max; if the seller will not accommodate these reasonable requests, I'd let the seller keep it, unless he's selling it for a very cheap amount, (< 10 EU) then it may be worth taking a gamble.