Anyone here have a knowledge of tax code re: bitcoins? could really, really use some help with a simple question

Hi there, I live in NYC.I'm looking to purchase a decent amount (50+ ideally) of BTC over the next four days. They're for a time-sensitive investment opportunity. I need them in my account. I made another thread that told me to go with localbitcoin because most exchanges won't/can't expedite the deposit/wire transaction.Here's my question: I have qualms about buying the BTC in cash because of the issue of tax basis. For example, if I meet up with a guy and give him cash for BTC, then it will just show up as another BTC in my wallet, but I'll have no record of paying the cash (other than through the history of my localbitcoin account, obviously). so if i were then to deposit the BTC in the future, wouldn't i be tax-liable for the entirely of the deposit into the account, instead of the small-to-medium profit i'm going to make with this trade?sorry if this was confusing - let me know if i can provide any other information. via /r/Bitcoin http://bit.ly/2evn6gA