Can I fill cracks myself before selling or renting out?
Cosmetic hairline cracks can be filled and repainted. Covering up a crack that is structural or active does not fix it and can hide information a buyer, tenant, MCST or PSI engineer needs. BCA's PSI guidelines expect engineers to remove plaster or finishes where needed to assess the underlying structure, and the PSI owner's guide states that remedial works recommended by the engineer must still be carried out if the building is sold.
From the RRD Field Guide, checked against the sources cited in Cracks in Walls, Beams and Ceilings in Singapore: Which Are Structural and When to Worry. General information, not advice for your project.
The fuller picture
Most hairline cracks in plaster, paint and at wall-to-beam joints are non-structural, caused by shrinkage and thermal movement. Worry when cracks are wide or widening, diagonal or stepped, tapered, recurring, accompanied by rust stains, sagging or sticking doors, or appear in beams, columns or slabs. Monitor with dated tell-tales and call a Professional Engineer for those signs.