As an alternative to the above suggestion, there is the ctrlr ini method. Check out
http://www.mameworld.net/easyemu/mamecontrolini.htm and docs/ctrlr.txt in your mame folder.
Personally I like this ctrlr ini way better, but it does have a little steeper learning curve. OTOH, the ctrlr ini files are supposed to be more future proof than the general input way, you can edit it outside mame, you can cut-and-paste lines and copy-and-rename files as needed, and works on all the games the "Input (general)" works on. (Neither work for games that have inputs hardcoded to non default vaules in the source, especially mahjong games.)
And to directly answer your quesion, yes, the seller is wrong.