I can actually think of an alternative explanation: small print press. He’d put out orders for various rare books with the traveling merchants who’d come through town, and every so often they’d turn up with one and he’d set about printing a hundred copies or so, then sell the fresh new copies to merchants heading towards various university towns like Avignon, Grenoble, Toulouse, etc. He’d likely keep a couple copies of each book for himself, generating a library, and might wind up with all sorts of books he couldn’t profitably make copies of to sell to the universities, like fairy tales.
If that’s his business model, then Belle might be the closest thing he’d have to an apprentice, since we can see he’s getting on in age and might have nobody else to even consider passing the business along to when he slips the mortal coil. As one final thought, her dad is an inventor, and might be the bookseller’s only actual local customer, which might also explain the relationship. Her dad would occasionally want certain types of books on natural philosophy, and the bookseller would be the one with contacts who could procure them. Just look how dangerous it was for him to go traveling all alone! Far better to leave that sort of business to professional traveling merchants.
If you combine these ideas, then you wind up with a bookseller who was training Belle as an apprentice for both small press publishing and money laundering, only to watch his very promising student be swept away by some rich guy the whole town was trying to kill twenty minutes ago. He admires her hustle, but it leaves a gaping hole in his succession plan.
Fortunately there are two newcomers in town, one with an eye for mechanics and meticulous attention to detail and one with a love of risks and charm to spare, and that's how the neurotic clock and slutty candlestick take over the legitimate and criminal wings of his enterprise respectively.
