Interesting question. Not noticed that but then not usually trawling sale sites for models unsurprising maybe.
i would hazard a guess though that it is because commercial sellers do their certification on the commercial basis and commercial certification cannot be mixed with the 'club scheme'. For example if you have a 10 year hydraulic from a commercial inspection you can't get your annual steam test and cold exam done by a club inspector, they will need to do their own hydraulic also. So it really depends which regime the purchaser intends to follow after purchase, if he wants to do the continuing tests through his club it makes sense to take the boiler untested and get the club to do from the outset of ownership. If continuing on the commercial basis then you may as well take the certificate. I am not sure who pays though because there is a cost involved ie if the seller tacks the cost of certification on to the deal it will save the buyer money, if the seller absorbs the cost then by not doing the test he increases profit!
Either way if taking a model untested and even if not, make sure it comes with previous boiler history as if you take your new toy to a club with no previous evidence of boiler manufacture standards and previous tests they are well within their right to refuse a certificate!
One last point is I have seen boiler certificates issued by sellers / in conjunction with the sale that do not comply with either standard commercial (10 year hydraulic) or club (2 or 4 year hydraulic) periods! One boiler in particular came with a set of certificates for six months and the details on the certificate did not properly match the boiler! Made me wonder why the tester was not confident to issue a proper full term certificate! A commercial test certificate should be backed up with an inspection report, particularly on a steel boiler with records of ultrasound readings and extent of inspection carried out. So as always it's a case of buyer beware and be sure what you are getting!
Paul.