For a simple calculation that should suffice in a stressful case interview, multiply population by disposable income/capita (or GDP/capita if you're desperate) and multiply that by a percentage of income that the average person seems likely to spend on retail every year (e.g. 33%?).
i.e. 300m * $40k * 0.33 = 4 trn
- which actually gets you pretty close to the ~4.5 trn spent on retail in the US last year. You'd get 5 trn from GDP per capita figures (~$50k/p.c.), so that would be an alright guess as well.