Has it always been possible to buy shares of NASDAQ listed companies on the NYSE? If not, what piece of legislation amended that?
Since NYSE merged with ARCA about 10 years ago. No specific legislation. All stocks may be traded on ECN's as well as their primary markets. The line that distinguishes them nowadays is quite obscured. The main differences are liquidity (create/take) fees, order types accepted, opening/closing auctions, etc. For the average person the differences are insignificant. However, for a high volume quantitative trader they may make large differences in P&L over time.