Paper currencies and cryptocurrencies only have value because people think they have value. Some currencies are backed by gold or other precious metals; others are backed by nothing but hot air. Even in this case, a currency can have value because people think it has value and use it as a unit of exchange.
This means that some companies want to exchange their goods for Bitcoins, and are ready to take coins for every item they have in stock. There are pizza places that accept cryptocurrencies and there are also some car companies that would give you a car for Bitcoins. There are shops that would give gold for Bitcoins. For example, Sharps Pixley, APMEX and JM Bullion. No matter the value in dollars, they will trade it for the same value of Bitcoins.
As mentioned before, there are pizza places that sell their products for Bitcoins. In the US, there is PizzaforCoins that will deliver pizza to a customers door (depending on where they live) in exchange for Bitcoin. Products are not the only things that get traded for Bitcoins. Some institutions accept cryptocurrencies for their services. Several private and public universities, as well as a couple of New York preschools, accept Bitcoin. Some legal and accounting firms also accept payment for their services in the cryptocurrency.
Of course, one can always buy happiness for others by donating to one of the bitcoin-accepting charities or crowdfunding sites, such as BitHope, BitGive or Fidelity Charitable. Basically, one can donate Bitcoins without limit.
For a list of the nearest offline stores that accept bitcoin, one can check an aggregator such as SpendBitcoins or CoinMap.
In conclusion, limits of buying with Bitcoins are set by the owners of products or services that one wants to buy. If someone trades gold for coins, one can spend thousands of dollars in Bitcoins.
