Definitely check your lease as some have specific provisions for car parking, especially if car parking spaces are allocated eg what can and cannot be parked there - must be taxed, insured, no commercial vehicles etc... There may well also be provisions about not running a business from the premises, at least without consent. It is possible there are also specific provisions about the visitor spaces so you can potentially rack up his breaches of the lease.
His premises are, I imagine, just his flat and maybe parking space (unless he has just a right to use the parking space) with the other land (common parts like hallways, stairs...) being the landlord's over which he just has a right of way which I doubt includes parking cars for sale so he is effectively running a business on someone else's land!
Could you make a log of vehicles there and length of time, tax status etc... and maybe get photographs without causing problems?