any tips, appartment is in a nice area, ground floor, well maintained building and grounds.

any advice

Valuation? By a professional? I’d be asking whether they’ve sold a unit in the same building and for what price, along with their commission rate/terms of engagement.
As to viewings by potential purchasers, get rid of most of the contents, bar essential pieces of furniture and put a few discrete plug in air fresheners around. Possibly get professional cleaners in for a deep clean prior to viewings. If the decor is a bit dated, consider a cheap redecoration in neutral tones.