What are these lines on the ceiling from?
Technically those lines appear where the ceiling joists are located. Depending on the building construction the joists may or may not correspond 1::1 to the rafter locations.

Those lines could be there for a number of reasons. Here are some I can think of:


There could be drywall joint lines under the ceiling paint that were not properly primed and the paint has been affected differently over time by the drywall versus the mud lines.
There could be an unevenness of ceiling surface that makes it reflect lighting in that pattern. This could be particularly pronounced if the ceiling material was cheap thin material.
This could be a somewhat porous ceiling material and over many years there could be discoloration due to material aging and dirt bleed through from above.
Similar to the previous item there may have been a roof leak or other flooding problem above at some previous time that has left water marks. Old dirt and dust accumulate over years and water comes and leaves dirty marks because of this.
Temperature differences, possibly caused by inadequate insulation, may cause condensation and/or staining in the spaces between the rafters


The main area of concern for you is to discern the cause and find out if the source of the problem is still present. This would be particularly important for item 4 above. If you really wanted to move forward to buy this house the best course of action is probably to make the purchase contract contingent upon a home inspection. Best is for you the buyer to contract for the inspection so it is not seller biased. From this you can be more sure if there is risk with this and you leave open the possibility to negotiate with the seller to make possible repairs or adjust the price to cover the risk potential.