Sounds risky. First off, I don't know of any dairy that sells a cow when she's ready to dry off - 7 months bred. If they've kept 'em that long they might as well keep 'em another 60 days, calve 'em out, and give them a chance as a fresh cow to pay their way.
Second off, if they ARE actually bred, there's gotta be a catch somewhere. If that cow is running through the auction she has something wrong with her; if it isn't obvious right off (feet), first thing I'd suspect is something internal. DA, hernia, that kind of stuff.
Then, how would you KNOW the cow is bred? The auction up here doesn't routinely preg check. Sometimes I can tell if a cow is OPEN - holstein running through with chalk marks on her rump is very likely to be a cow that was open a few too many times. Saw a fat, dry cow go through the other day - either she had problems at one point, and she was dried off and fed until she looked good, or, she was bred and just slipped that calf and whoever owned her said "Enough!".
And...how would you KNOW the cow is a 2, 3, or 4 quarter cow? Never heard anyone announce it when the cow's going to slaughter anyway. To put it plainly, udders are what I deal with on a daily basis, and I wouldn't dare guess on a cow running through the auction if she was a 4 quarter cow or less. My boss has some cows running through the herd with a visually balanced udder...that only milk from 3 quarters. I'd be willing to guess if she'd had mastitis at some point, but not which quarter or if she had it currently. And...it doesn't matter to the folks buying slaughter cows at the auction if the cow only milks from 3 quarters. Doesn't mean they'll pay more or less for her. It's based on body condition.