As said before, its a manufacturing imperfection which doesn't look like its going to be an issue. Wood is a very natural material and imperfections are to some extent to be accepted. Like others have said, if it was a high end £1000 + bit of kit maybe it shouldn't be acceptable that it left the factory that way but if a i sold a bass with a tiny mark like that and then the buyer wanted a refund I would agree but feel a little aggreived. That however is just me and these things are subjective. If it really worries you then maybe offer to return at your cost if the seller gives you your money back.