Tell him to try them on first if at all possibleGeneral piece of advice (almost a rule, it's so universally true): IF your friend wants to get a deal buying boots which he's never tried on, on eBay....then, if he's smart, he should know what size fits him. The only way to do this, really, is to try the boots on. Even here, the potential boot buyer, especially if he's inexperienced, may get an overload of information. For example, as many of us know, more experienced skiers tend, as a rule, to size their boots more snugly - so what for a beginner might be an uncomfortable tight fit...for an experienced skier might be almost too loose. It depends on your preferences, your experience, your foot size and shape (different boots fit different foot sizes in different ways). Plus, different manufacturers sizing/fitting systems vary wildly, in spite of the fact that they all claim to use the standard mondopoint system these days...
How does this all apply to your friend? Simple. If he can, he should go to a store or retailer where they have different sizes of Rossignol Soft boots in stock - and he should try on several different sizes...and try to figure out what works for him. If the store doesn't have any Soft boots in stock, then he might consider trying on other relatively beginner/intermediate Rossignol boots, for sizing purposes (on the theory that if he can figure out his correct Rossi size for one boot model, it may hold true for another Rossi boot model - a theory which works some of the time...but not always).
Once he's done all that, then he'll have a good idea of what size boot to buy. He should also have a better idea of how much Rossi Soft boots really are selling for. Stores may have end of season sales which will be better than many of the prices on eBay - something worth checking out - and if you can support a good local ski store, it's always a good idea.
But if he doesn't do some or all of the above, then, literally, he's GUESSING if he buys boots off eBay. Sometimes people get lucky when they guess....other times, they don't. My personal theory is that boots are one of (if not the) MOST IMPORTANT pieces of ski equipment anyone can or should buy. Especially for new skiers. If your feet don't feel comfortable or good than skiing can be a nightmare. This is why many recreational or part-time skiers have embraced the so-called Soft boots (made by Rossignol and other manufacturers), even though more advanced skiers judge them as being too 'soft' for real performance.
By the way, I belong to the category of advanced skiers who value comfort over painful fit for performance purposes, and I bought Rossi Soft boots for myself. I have nothing but good things to say about them.
P.S. Soft (or softer) boots are also favored by many if not most of the newer generation of young, hip twin-tip skiers who congregate in terrain parks - and who realized, awhile back, that they don't need or want the so-called performance features of many higher-end (and usually tighter-fitting) boots, many of whose features were designed to help racing skiers ski faster. Well, guess what? A lot of us aren't racing skiers...and instead of skiing faster, we want to ski more comfortably. This may be a heretical opinion - and one which is in a minority on this board - but there you have it! Tell your friend good luck!