Opinions differ on what overfeeding is and how bad it is or isn't for the snake. If it starts looking like a pinhead (it's body outgrows it's head) it might be time to slow down a bit.
By the way, is this a female or a male that you are looking to buy? The general consensus nowadays seems to be that a female should be at least 1,500 grams before breeding. There have been reports of rather short and stocky females breeding much lighter and some rather long girls may need to be 2,000 grams before they have enough fat to produce eggs. Males have been known to breed under 400 grams (presumably rarely) but it seems like people have better luck with 500 grams and up with perhaps 800 grams and up being most likely to be successful.
I picked up a 01/04 male back in May and I think he was only about 170 grams when I got him. Presumably he was on a maintenance diet at the breeders for the first 4 months of his life. He apparently is hungry now as he even eats in shed and is 450 grams now without me trying vary hard (weekly feedings of not particularly big meals). He is a possible double het VPI snow and I'm planning on trying him on a couple possible het albino girls here in a few months.