The Greeks a seller actually watches: theta, vega and gamma on expiry day
Sellers get paid in theta and pay in gamma and vega. Understanding the exchange rate between them is the whole business.
Theta: what you earn
Theta is the daily decay of an option's price. It is not linear: an ATM option loses far more in the last two days than in the previous five. That is why expiry-day selling exists, and why it is crowded.
Gamma: what you owe
Gamma is how fast delta changes. In the last hours of expiry an ATM option's gamma explodes: a 100-point move can turn a comfortable short straddle into a directional position of several lots' worth of delta. Theta and gamma are the same number seen from two sides; the more you earn from decay, the more a move costs.
Vega: the surprise
Vega is the price sensitivity to a one-point change in implied volatility. On calm days it is a friend to sellers; on event days IV can jump five points in minutes and every short option marks against you at once, before spot has even moved.
Measuring the book, not the leg
Per-leg Greeks mislead. What matters is the net: delta in rupees per 1% move, gamma in rupees per 1% move, vega per point, theta per day, for the whole book across expiries. The terminal's RISK board shows exactly that and tells you how many futures lots or ATM options would flatten the delta.
Sizing
Decide the rupee loss you accept on a 2% move before you sell anything, and size so the shock grid says you are inside it. The VIP Indicator SELLER engine only prints a strike when the regime is calm, but the position size is always yours.