So, $50/year and $3/day, one day per week, 52 weeks per year. That totals $206, by my calculations, or about $18/mo.
If they sell all 7 shares, they're getting $1,442/year out of one goat.
I dunno...I've heard of selling shares of animals before, but this sounds a little sketchy to me. If she's giving, say, three quarts a day, that means you get three quarts a week.. Three gallons a month, roughly. Even if she milks through -- an entire year -- and gives an average of three quarts a day, you're still looking at paying about $6/gal for milk. If she milks 10 months, you're closer to $7/gal.
Something else that's mildly to moderately worrisome part is the mention of "fertilizer" and that you get "whatever the goat produces" on a given day.. Are they saying everyone "gets" to take turns coming to get their day's share of 'fertilizer?' Because, to me, 'fertilizer removal' -- a fancy term for STALL MUCKING -- should be part of the care you're paying the $3/day for.
All in all...I wouldn't touch this with a 10' pole, even if I didn't have goats. But that's just me.
It is a somewhat funny way to word it, but this person is selling you a share of the goat so that you, as owner, can legally get raw milk. Around here it average cost per gallon of Jersey milk is between $5-10 dollars per gallon and around here it's a herd share, not part of just one animal. Sounds like this person is new to CSA's. Craigs list ads are notoriously bazaar in their wording. If it's of interest to you then go chat with them and shop around to see if their per gallon/quart price is within market parameters for your area.
Considering that around here people pay up to $6/QUART for raw goat milk, that doesn't sound too bad. As far as the fertilizer goes, perhaps they put that in because some people would want a share of it (I would take it for my garden...). When we had goats, we often had friends who would ask if they could come by with a bucket and get some poop for their gardens.
I imagine the fertilizer is a bonus. I mean people usually give soiled chicken/goat/horse bedding away for free on CL because people will take it for their gardens. As someone else mentioned it's just a goat share which is a fairly common way for people to buy raw goat milk. I think their pricing looks very reasonable as I do see raw goat milk going for $8 a quart around my area. I'm also pretty sure they have a dairy goat because you wouldn't be selling 7 shares, 1 for everyday of the week, of your goat if it wasn't producing well everyday.
ETA A breed like a saanen could produce a gallon or more of milk per day during their peak.