Yes, it is definitely a grafted tree - and that lump is the graft union. Not a big deal if you put it in your landscape, where you won't see it at all once the tree has a 6" trunk caliper. However with bonsai it is very difficult to hide.
Additionally, it is important to understand what
@hinmo24t is saying. If you cut the tree back to
below the graft scar, you will end up with a green Japanese maple. The more valuable top of the tree - the cultivar stock - will be cut off and thrown in the trash. If you are buying the tree because you like how the foliage looks, you will lose it.
Many people who buy grafted Japanese maples for use as bonsai buy them for propagation purposes. They grow the tree in their landscape, and take cuttings from the branches, or air-layer thicker branches to get larger material. Or, if you find a Japanese maple that has a really chunky base, you may simply cut it short and be ok with it having green foliage. However, make sure you aren't paying a premium for a rare maple cultivar - and then throw it in the trash
The image below is someone using a landscape Japanese maple to air-layer off branches to become individual trees. Each new tree will be a clone of the parent cultivar (the top of the tree) while having its own roots and no graft.