Any bike could be stolen, its the pit falls of distance buying, off roaders can have a hard life so at least scan the piccys well for signs of dropping but twisted forks for instance dont show in photos.Check the sellers feedback on sales.
If you can see it first It will give you a chance to check V5 against the bike numbers and not sure when you were inteding to do a HPI check but it must be done before you buy after the event all that will happen is either you will then try to sell a bike you know to be sttolen IF thats the case or you go back to the seller who saye--is it? sorry about that didnt realise--- police wont do him if they think it was sold in good faith but they will take the bike from you EBay I beleive dont cover vehicle sales.So you will lose your money.
If the V5 is genuine ask for photos of the V5 showing the frame/ engine numbers and reg no.And photos of the frame/engine numbers, number plate waste of time but you can do a DVLA vehicle check with the reg number which I dont think will say wether stolen or not but will say what the bike should be.
If you are buying a 2017 model your not saving that much over maybe a new one or a 2018 still in a dealers (big discounts on last years model normally) just a thought.
If you are buying a off road bike for the first time and not off roaded before get a sensible size one 500s have over 60bhp and on the dirt that is a lot of power on wet roads on knobblies they can be leathal spinning the back wheel, in the dry they will flip over faster than you can say-- fuck this is going to hurt--300 two strokes probably just as bad.