Well as a hobbiest, I try and research to find the best reviewed provider, but I don't think the provider needs to "provide proof", if she is reviewed well and a good one then I am on my way, too many picture collectors and just not concievable, IMHO!

Sounds like a term that would be used for gentlemen circa 1950.

Newsflash:  There is no 'trade', there are no rules and even little common understanding in this 'business' replete with suckers and scammers.

Suggestion:  Ask for a picture in a manner that does not convey you are doubting the information in the ad and require proof.  And even if you get one, it could be fake, so your best bet is to check reviews.  Even with reviews, a bit of skepticism may help.  There are sycophants posting reveiws, who hope that ingratiating themselves with providers will get them special benefits.