When purchasing a boat the common advice is to have the seller pay to fix whatever needs fixing. The seller can pay for these repairs directly, i.e. paying the yard to fix them or indirectly through a price reduction, which may have been taken when the boat was initially priced.
In this case, if the asking price of the boat was lower than similar models, then the price reduction has already been taken and missing halyards is not a hidden defect, it should be obvious. Price adjustments after an offer are only for hidden defects, ones not immediately obvious to the buyer.
For something like halyards, I would not ask the seller to replace them. The seller's incentive is to get the job done as cheaply as possible, which means using inexpensive and probably stretchy line for the halyards rather than an upgrade to a more appropriate line. And small extra costs, such as adding a reeving eye to the halyard or replacing the shackles would not included.
If you should make an offer on this boat, factor in the cost of the new halyards and the cost of installing them and replace them with the line you want, not the cheap stuff the seller will provide.