DNIM here's how it works.
If at the end of this year you owe 1K or more of taxes on your personal tax return you're required to start making estimated tax payments next year. There might be a penalty involved as well for this year. Estimated tax payments is not as simple as 30% of every sale. There are types of sales and deductions that have to be considered.
For example sale of part out bricks is an ordinary income and taxed at your tax bracket. Sale of sets held for more than one year could be long term gain on investment which is 15% or could be 28% sale of collectible or it even could be ordinary income depending on how you want to interpret IRS rules.
Now deductions, your storage space even if it's in your house is deductible, your wear tear on the car is deductible as it relates to your business, packing supplies, phone, printer ink, internet connection, etc etc.
The proper way to do it is not at the time of sale but rather at the end of the period, based on the proceeds and expenses of the period. Based on what you are saying I would say that you should segregate yourself from your business at least financially if not legally. My suggestion is to have two business accounts totally separate from your personal ones. One account would be your operating account, the other would be your tax liability holding account you would transfer funds on a monthly basis from operating account to liability account depending on your calculations. You don't use your business accounts for anything personal, i.e. personal bill payments, vacations, etc. You withdraw money from operating business account to your personal one and then do as you please. Also I would get a separate credit card just for the business expenses and pay it out of the business operating account. I would also say that if you made 10K last year from your lego business you go LLC route and completely divorce yourself from your business.
The only per sale tax that you should stash away, right away is the sales tax that you charge your customers.