Jabba values the droids like people, not equipment.
Jabba has a severe staffing problem, and Luke recommends the droids as good staff
Jabba is frustrated, professionally embarrassed, and at his wits end because he can't find competent staff with the right qualities (More on this later) Since individual droids have fundamental personal qualities (more on that in a bit), and Jabba's staff have demanding jobs, (and Jabba is picky and temperamental) he needs the right individuals, not just certain models of droids. He hasn't been able to find individual droids that suit him. The Droids excite Jabba because Luke testifies to their suitability, suggesting they might solve his problem:
"Both are hardworking, and will serve you well."
On Tatooine, the droids are valued only for their menial labor, and their personal qualities are ignored.
The Jawas could've taken the time to see that R2 is the feisty, quick-thinking lion-hearted hero he is and found a better buyer and a better price, but they just wanted to make the sale quickly. Owen doesn't interview them (even cuts off Threepio's elevator pitch) because most any individual can do menial farm-hand work, or can learn quickly.
Droids have individual qualities
Everyone on earth could be considered to be a 'Human MK1' model of creature. We are all the same species, the same equipment. Yet individual humans are of very different value to a master. When slaves were traded in earth history, they weren't all valued equally. Even among two men of equal physical strength, one with a reputation for hard work and years of experience would be valued much more than one with an insubordinate history and no applicable skills.
Droids of the same model have shown differences in ethics, temperament, personality, and mental proclivities. Furthermore, there's evidence that these traits cannot be arbitrarily reprogrammed. When R2 quibbles with Jabba's Chief of Cyborg Operations, EV-9D9, this line happens:
"You're a feisty little one, but you'll soon learn some respect"
Note that she doesn't say:
"Your personality has drifted out-of-spec and become feisty. I'll just run this Perl script to make you obedient."
A droid's temperament cannot be changed easily. Like a person, individual qualities of a droid make it more or less valuable to particular masters. And that line brings up another point:
Droids can learn
Which means that some droids are better suited to a job because of experience. Knowledge and skills aren't always enough. Real-world experience is important. Like a person, a droid is worth more than another of the same model if it has better experience.
So like a person (or slave) the suitability of a droid for a particular job depends on a droid's individual characteristics and career experience. The droids were of average value on Tatooine because no one realizes or cares about their individual potential. But for Jabba, who's trying to run a crime syndicate,
"A good droid is hard to find"
Jabba wants the droids to serve in his court, which is where his friends, business associates, and enemies (not mutually exclusive) come play politics and organized crime, sometimes with a thermal-detonator in-hand. Consider a King hiring a courtier or Al-Capone hiring an assistant. Even among the people with the right experience on their resume, it can be hard to find a good fit. Many personal characteristics—confidence, composure, temperament, demeanor, shrewdness, empathy (or lack of)—must be finely tuned to do well in a position like that. You need someone with enough experience to handle sensitive politics and work under pressure, but you also need someone who's personal qualities reflect the values (powerful, organized, ruthless) of you and your business when facing your space-mob frenemies.
Jabba's has a staffing problem both chronic and urgent
He hates most of his current and former staff. The last interpreter was disintegrated and there are many other droids being tortured in the dungeon presumably for pissing him off. The only droid that seems comfortable in the space-cartel trade is their torturer, and Jabba's Chief of Cyborg Operations, EV-9D9, who has the rare qualities of sharp authoritarianism, ruthless professionalism, and wry sadism. Most droids, like most people, probably don't have the right personality, ethics, temperament and guile to work in the court of an irritable hedonist space mobster. Although he can probably afford any model of droid many times over, he's having difficulty finding the right individuals.
Jabba's constant dysfunction with his attendants is a disaster
Jabba loses his composure and rages at his helpers in full view of the court. The seedy clients and spies see the dysfunction and circle like vultures. Jabba isn't even sure he can trust his repugnant majordomo, Bib Fortuna. Jabba is at his wits end trying and failing to find the right people, destroying droids and even killing dancers in exasperation and rage. Dysfunction makes him appear angry but inept. This is all a bad look for a crime syndicate and he probably knows it.
Luke seems to be offering the cure to Jabba's most enraging and embarrassing problem.
It's possible Luke knew about this situation. But even if it was a lucky guess, the droids seem like a perfectly timed salvation to Jabba's woes. This chronic problem has just come to crescendo now that he's disintegrated his last interpreter and has an urgent vacancy. Now C-3P0 and R2 don't actually have the right personalities either (3P0 is awkward and easily flummoxed, R2 is too feisty) but Jabba doesn't know that right away. Jabba is irritable, but also excitable. When Luke says:
"Both are hard working, and will serve you well"
He gets excited because Luke seems to be saying these droids have the right qualities and experience. It seems plausible because they have been serving a Rebellious politician and a powerful Jedi. They could be steely, shrewd, rebel-trained agents that will help him regain control of his court.
As the idiom goes, "It's impossible to find good help these days".