As best I can tell, modern forgeries are not a problem for this type (variousy called an Ort, or 1/4 thaler) from Sigismund III (there are lots of forgeries of Polish Thalers, and on some rarer 1/4 thalers from later kings).
For that budget, you are likely talking about either an example from the Bydgoszcz (Bromberg) mint 1622-24 with the 3/4 length portrait of the king and Polish-Lithuanian arms reverse, or 1623-26 from the Gdansk (Danzig) mint with the shorter bust, and the Gdansk city arms reverse.
The latter type were often better struck, but I think tend to be a bit more expensive.
Other than eBay, I can recommend the weekly (Thursday) auctions at WCN. Their commission on these auctions is only 10%, they accept paypal (for an additional percentage), and they ship by registered mail for 25 PLN. You may get a better coin for your money there, though opbviously you need to take the fees into account. Here is a link to a search in their archives for 1623 which is a common year for both types so you can get a feel for selling prices there https://wcn.pl/archive?q=polska+ort+1623
Just wait till we get to the Renaissance portrait and sword-wielding eagle on coins of Sigismund I in another month