Having only recently started buying and selling music gear on Reverb.com, I have been struggling with a... shall we say "feature omission"? When a keyword is typed into the search field, the results are retrieved based on Seller Location of "Anywhere". To filter to a specific country it is necessary to click the "Change" link against Seller Location, then click the dropdown field to display the options, then select the country from the dropdown list (in my case having to scroll waaay down to "United Kingdom").
So that's three clicks, which you have to do every time you change the search text. I quickly got so annoyed by this that I messaged Reverb to... well, complain. I did get a fairly prompt and coherent response admitting "There isn't currently a way to modify your settings to always only show listings from UK-based sellers, but that is a feature we may offer in the future."
However I may have come up with a workaround using the bookmarklet technique. By creating a bookmark that has the URL of
all that needs to be done is click that bookmark after entering a Reverb search to apply the region filter. The appropriate filter field seems to change value accordingly as soon as the bookmarklet is run.
If your desired region is not GB, apply the filter manually then check what the query parameter value is in the URL and insert that into the bookmarklet value in place of "GB". For instance Afghanistan appears to be "AF"!
Be aware that no liability will be accepted for any bad things that might happen as a result of trying it yourself!