This starts to matter later in the round when you need to buy as many mercs as you can before merc pool runs dry (especially during market hours).

click the dropdown tab for lvl1 mercs, hit the 1 key twice to set it to 10 mercs

Press tab to go to next merc, repeat if you want to buy lvl 2 and lvl 3

then press whatever number is required to validate

for example if you want to buy 10 of all...

mouseclick 1 1 tab 1 1 tab 1 1 tab [validation code] tab enter/return
(select merc 1 queue 10 next queue 10 next queue 10 next validate next purchase)