An "eladó" is usually a sales person, sales clerk. Or the seller in a transaction.
A "vendor" is a bit different. For a street vendor, I would say "árus". Street vendor - "utcai árus", cotton candy vendor - "vattacukor árus", hot dog vendor - "hot dog árus".
In business terminology, the vendor is "szállító".
A vending machine is "automata". If it is a coffee vending machine, then "kávéautomata", etc.
"Eladó" is also used for something that is for sale. You can see it on signs:
"This house is for sale" - "Ez a ház eladó".
'salesclerk' has been suggested by the users of this course on countless occasions. I do agree with you, however, that this is not exactly the same as 'eladó', just too close not to accept it. Actually Duo accepts many more solutions.
I've changed their order, thus at least 'salesclerk' is not the very first now. Thanks!
What you see on the billboards is short for 'Ez a ház eladó.' ('This house is for sale.') Here 'eladó' is an adjective, and actually the predicative of the sentence. In the example above 'Az eladó ott sétál.' the word 'eladó' is a noun (and the subject of the sentence).
Cf. the word cooling, which can be either an adjective or a noun depending on its function in the sentence.