if out of those 30,000 people (assuming that is all people of all ages), then that would be about 10,000 families, so 10,000 'kids' of which maybe 5,000 would be of PC buying age, of which perhaps 1/10 would be interested at an enthusiast level, the other 9/10 perhaps in a need pc for school basis. Of the family groups there's perhaps 50% of them that care enough to buy things.
So thats 500 enthusiasts at perhaps 4 'needs' per year, and 4500 users at 1 'need' per year and another 5000 families at perhaps 1 need per year.
So thats 2000+4500+5000 = 11500 needs per year. How may shops in your area? 3? so assuming an even split (steady state in 2-3 years time) thats abut 4000 visits per year, assume a split of 50:50 between small and large sales, small sales geneating $10 of profit (profit is all that matters turnover is irrelevant) large sales generating $100 of profit.
So $20,000+200,000 = $220,000 of profit per year, as a potential. Divide this by weeks in year, = 4400 profit per week, assuming that this is at a profit rate of 20% then you need to be carrying $20,000 of stock to service this assuming you only have in stock precisely what they want. If you carry 10 items for each item that you sell then this might be $200,000 of stock, and therefore at least $200,000 of startup capital, interest (not capital) payments on this would be 20,000/year at 10%, which is about 400/week + repayments, all of which would eat into the $4,400 profit.
So I suspect that after rent, interest capital repaymemts, tax etc. you'd be ending up at around $2000 profit/week. All of which could be your income. However it only takes the sales assumptions above to be out by a bit and that could become just $1000/week. I'd suggest you model the fixed costs vs variations on the income and see just how marginal it is.
If you want PM me with your email addy and i'll see if I can knock together an example, with some variables tht can be changed.
I'm behind you on this, but i'd hate to see you go into it not understanding the likelihood of there being an issue.