Anonymous
Anyone can "claim" it, but a true b.l.t. would never refer to himself as a "body language expert." No one could possibly take you seriously if you did.
Cogito
Calling yourself 'an expert' on any subject, unless you have very high qualifications and many years of experience in that field, would only make you sound like a conceited kid and few people would take you seriously.
Anyone can claim to be an expert on any subject at all - but if you were planning on taking money from anyone for your services or advice as 'an expert', you may well be committing a number of crimes - false representation, fraud, theft, etc.