I'm not sure there are any "risks" to buying a too-long bag (other than the economic risks Mouse pointed out: $145 for a good bag that's shot is not money well-spent.) That is, if the bag is in good condition, and you're aware that you probably can't push it to its EN limits any more, you wouldn't freeze in it. You'd carry more weight than absolutely necessary (unnecessary weight is not fatal, despite what you read from some people here, including me.) You will spend more energy warming it up, unless you do as Mouse says and fold it under or tie it off (or find someone who can shorten it for you.)

But, if the price is right, the condition is good, and you don't mind not having the latest, greatest, lightest - and most expensive - bag out there, it sounds like it would work.