I’m not doing up a house to sell, but things that I think make an immediate difference when I do them at home are:
Clean porch - leaves swept out, no cobwebs, polished letterbox and bell etc
Big pot of something bright by the front door which you can take with you. Maybe an evergreen at the back (small like conifer or white flowering skimmia or something) and then a few things in flower at the front. Good right now, if you haven’t planted bulbs, are primulas (all one colour look nicer I think in a small space) and when they’ve finished, you’ll be into bedding plants after last frost
Welcoming doormat - hug at home (online) do great ones and some are washable
Coats and shoes all put away
Neat throw and plumped up cushions and sofa cushions
Small stacks of things that look nice to do or read on the coffee table - books, small games etc
Lovely smells all the way through - lemons are cheap and smell great
Fresh flowers aren’t expensive now - daffodils £1 a bunch, irises are usually quite cheap at Tesco and freesia will make a room smell Amazing
You could frame lots of small things and make a collection of them, using the same colour frames (paint them if you need to. Cards or postcards are good for this and cheaper than prints
I also like a big clock instead of artwork and you can take it with you. Mirrors the same.