I'd only get stuff I'd use in my next house - charity shops, facebook marketplace and high street shops (wilko, dunelm, next, home bargains)... You have to pick and choose between what's tacky and what looks nice though!
Obviously you need the house to look good for viewings, but I think the house needs to look the best for the photos (easier to hide stuff for photos than for viewings too!) as people will spend longer looking at them and keep referring back to them when deciding which houses to view. plus neighbours who nosy will see this
Cushions - nice cushions that compliment each other (so a few plain and a feature cushion or two). Chosen wisely, they can bring together elements of the room (sofa, curtains, rug). New cushion pads if yours are looking a bit sad (Dunelm for a few pounds).
Candles - chunky church candles either on the hearth or on a large mantle piece if it's it's looking a bit empty.
Plants - I have a decent array of house plants, but some are looking a bit sad. I'd chuck those ones out and replace with some cheap ones. Ones that trail look good on shelves/sils. Pots that compliment each other, looks nice to have a few together in different size/height pots. Also plants for by the front door, something seasonal like daffs, you can get ready made tubs which don't last long as they often put plants that grow too large in them like conifers, but will be fine whilst selling.
Rugs - mine are alla bit sad looking (kids plus food!) but you could get a couple to add a bit of warmth to rooms that are put out for photos/viewings so they keep nice (esp if you have children!)
Mirrors- little cute mirrors in dark corners, make sure you have one over mantle pieces and above bath room/cloak room sinks.
Photos - nice frames - not loads of personal photos but enough to make the house look homely. Free standing ones look nice on shelves, on top of side boards/tables. You could always buy some paint and paint an array of old wooden frames to match.
Pictures - you could always get a few large canvases (I'm sure they had loads last time I went to The Range) which will add a feature to an empty wall.
Boxes, baskets and bowls - soft (eg knitted or fabric) or natural boxes or baskets (eg wicker or wooden) to keep nick nacks in (I have some in my bath room for cleaning bits and sanitary ware). A wooden bowl full of fruit will always look good in the kitchen /dining table, and maybe shallow but wide ones with pine comes.
Flowers - buy a few days before so they are opened up (but not losing their petals!) - you don't need a big bouquet, instead something that is seasonal (eg daffodils now or sweet williams later in summer) and cheap will look nice and full once they've opened. Make sure you out fresh water in... Or put in opaque/ceramic cases/jugs to hide the water when it goes a bit slimy.
Blankets - place folded over sofa / arm chair arms (esp good to hide if there's a hole or a stain!), again can add colour to the room and bring elements together. One over the end of the bed can look nice too and add some colour, especially if your house is quite neutral.
Bedding - plain if patterned curtains (white look good), but if the room and curtains are plain if will look nice to add some colour and pattern, making it a bit more homely.
Lamp shades - mine are all a bit grey and dusty and can get some nice ones cheap ones that again draw the room together. Also floor or table lamps if you don't have any already.