Strings occasionally break, be them new or old. Replacements are truly unnecessary on pianos unless a piano is going through a restoration, or the stings have lost some zing.
Sticking Key Repair:
Sticking keys is normally not a major issue and the cause tends to stem from humidity issues. Excessive friction from grime, swelling, corrosion, lack of (piano) use, or objects stuck in the movable parts cause sticking keys.
Inside all pianos lie thousands of mechanical parts, known as the action. In order for the parts to move, they need conditioning & lubrication every so often. This procedure of conditioning is called “friction reduction”.