A "significant purchase" will vary based on the town/metropolis, how involved the Guild is, and whether the game world has 'magic shops' set up like BBQ Turkey Leg stands at the Ren Faire.
Even just hanging out in the local Tavern may let the secret slip..perhaps paying with an out of circulation coin.. leading to speculation that you have had access to ancient treasure.
In the case above, Crothain mentions it comes in Gems.... not the most portable for small scale purchases. A sudden influx of quality gems into a small time market will be obvious, and a big time market will have a fair amount of Guild involvement as the prefered means of cleaning 'surplus' income. It would be best for the party to slowly sell the gems over a long period of time, using them to maintain normal living expenses. Dropping enough to grab magic items, even at 'Magik Shoppe' styles, should attract Guild or freelance attention.
If the game is not is a 'Magic Shoppe' style, then its even more noticable as they need to do Gather Info checks to discover the mage that has the item or ability to create the item they want.
Of course.. all this depends on your game world. Mine tends to channel Sanctuary and Lankmar in this regards.. with a coloring of the Black Company. The Guild would know that you have surplus funds in a few days after hocking any of the gems.. and if you had a Rogue in the group, they would be aproached to pay thier dues.
Of course, YMMV. You could completely ignore the economic impact on the DnD landscape.. much as most games and campaigns do, and minimize the greedy folk that populate your world. In which case, balancing the sudden boost in power should be a reduction of future gains until the wealth level is more in line with your gaming style.
Buying a ship would be cool
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Yup, definatley turning into a DnD economy primer

There are lots of variables..and so many of them depend on the game style.
The big cash influx can be used to make the game really cool , complex and alive.. or turn it into flat playing magic equipment carried by 2D beings. All depends on the GM and the players.