1) The BTC market isn't that liquid (compared to, say, USD-EUR forex). If you want to buy/sell X BTC on behalf of your customers right now, you may be unable to do it quickly in that amount. So, you can't really wait until the final confirmation of 'money from buyer' and only after that start to do things - you may finalize the deal at that point, but you'll do some buying and selling before that simply in anticipation of the deal coming on. You can and should manage the open positions so that they aren't too big in any one direction, but generally you'll have to be part market-maker and have reasonable non-tiny reserves in both USD and BTC just to do your daily trades; otherwise at times you'll be unable to make deals and have to cancel/fail/postpone them which hurts your service.

2) 'When you receive money from buyer' is a tricky issue. In general, you won't get the money from the buyer when you want it and in the full amount. No matter if you accept credit cards, ACH or whatever - some of them can be reversed, some of them will be reversed, and again, noone will simply trust that you'll return the money when needed, so they'll freeze or hold some of that money for some time. Well, unless you can prove that you have so big reserves that you don't need the money anyway.

3) As you don't have any reasonable reserves/trust/credit rating, in any relations with other financial institutions (such as paying out funds) you'll be required to either cover everything upfront, generally a day before (which is expensive to you), or hold deposits/bonds/whatever as collateral (which requires the collateral); so right there you'll have to hold reserves comparable to your daily turnover. In order to get a dollar to customer on Dec 15, you'll have to give away the money some time before that.

4) The same holds if you want to integrate in any of the payment systems worldwide to receive and send money - if you want to get direct access (instead of extra price+delay by going through an intermediary), expect to put up multimillion deposits simply to ensure that noone else loses a dime in any case. The general principle is that counterparties occasionally tend to go bankrupt or act fraudulently, so everyone extends only very limited $ amount of trust. This, coincidentally, causes almost all of the payment processing time in the 'classic' banking - you can send the payment cheaply in seconds, but it takes a few days to do this in a manner that ensures that one bank doesn't lose money if another doesn't pay, either intentionally or because it's unable to.

5) The financial partnership deals you get depend on your size - the bigger you are, the safer you are perceived; the smaller you are, the more you pay for the same things. And I'm not talking about $25m-capital-small - small means a tiny local bank or 'pocket bank' of some person or company with $250m+ in assets; 25m capital means you don't even get to talk about non-standard terms for servicing your account, much less get them.

And there are other issues. It all adds up - each such item holds up some funds, so in general, you need a sizeable capital that's proportional to your total turnover; and the full turnover of USD->BTC->USD, not just your revenue.