In Chrome and Safari I disable external URLS for iTunes and the Mac App Store to avoid the apps opening every time I look up an app. In Safari I use No More iTunes, and in Chrome I just disable external links of those types using the built-in security tools. However, I sometimes do want to open iTunes or the MAS, which can be cumbersome — especially in Chrome — if the process is disabled.
This is a companion discussion topic for the original entry at https://brettterpstra.com/2013/01/22/services-for-opening-itunes-mas-links