I'm not sure it's 'most' he does have both OEM and aftermarket however.
The parts I've bought so far are all OEM. I get the # from BRP parts site then search that site for them. they've come in the OEM sea-doo packaging and the parts as far as I can tell were all OEM, from screws,to hoses, to pump bearings and washers...
My local dealer (45 min drive) is always $10-$15 shipping... this place is usually under $5.00 with USPS tracking. that's a deal to me!