Want to respec my lvl 37 engineer from cannon to melee. Will I be flagged as a cheater if I consume one?

no only if you create one

Does anyone sell these for igg or fg?

you can make them yourself.


copy save folder
enable console (settings file)
login single player
use console to create potions.
put potions in shared stash.
go into the save folder you just played (not the copied one containing your original char)
cut the sharedstash.bin
paste it into your copy save folder
delete old save folder
rename copy save folder back to save
log in to internet and take potions out of shared stash.
grats