So as the other two answers have explained, "...in the market, as it were." is Jack suggesting he is "shopping" for a ship, "in a way", or "as it currently stands".
But the other answers don't really explain why this line of dialogue is quite funny.
First off the phrase 'in the market' is a sophisticated and business-man like way of saying he was shopping for a ship. If one noticed, many of the other pirate characters, such as Marty, Pintel, or Scrum don't have the best vocabulary, vocal skills, or social skills, kind of playing on the stereotype of people with a poor to average education level. Jack however tends to have a rather extraordinary vocabulary and vocal delivery, making him even more of an odd-ball that helps Johnny Depp be more comedic by contrast.
Another reason this dialogue is bit humorous is because, like he had suggested by tacking on the, "as it were", the audience knows he's not actually going to "buy" a ship, he's of course going to steal it!
Dead Man Tell No Tales Transcript:
JACK SPARROW: Am I not the Captain of this ship?
CREW MEMBER: You call this a ship?!
But on top of it, the fact that he wanted to reply at all and acting as though he has the best of intentions, also could be seen a funny self defense mechanism and showing a sense of pride, by deflecting his vulnerability. Later in the fifth film, Dead Man Tell No Tales, after the bank robbery goes terribly wrong, he has a ship, but it's nothing special, as the Black Pearl remains in a bottle, but the crew uses the pitiful-looking ship as an additional excuse not to sail with Jack any longer, further demoralizing his status, which contributes to the betrayal of the compass. "We no longer follow a Captain without a ship!" So for pirates, ships matter!