It's illegal to sell kit (or anything else) anywhere (including eBay) if it doesn't belong to you. You may be breaking the law buying stuff that doesn't belong to the seller or that the seller doesn't have legal authority to sell.
Issue kit belongs to the Queen, therefore it shouldn't be on sale on eBay.
Of course most issue kit is legitimately available on the open market and there is no reason why the owner should not offer it for sale on eBay.
MTP and PCS are more of an issue because it has not been released to the market and whilst it's possible that some of it may legitimately have passed into private hands - legitimately issued and legitimately not been required to be returned - one would might reasonably speculate that much of the MTP on eBay doesn't fall into that category.