That's a good question.  AFAIK (and I'm a newbie too) it would not affect trading prices.  But it is possible to look up transactions on http://blockchain.info/, and I would think active traders would monitor the volume of large, anonymous transactions just to get an idea of how 'liquid' the market is. 

I agree 100% about private sellers.

FYI - regarding bitinstant.com, I just used them to buy BTC. I filled out the form on the website, walked over to my local CVS, spent 10 minutes going through the process of calling Moneygram and paying at the cashier. By the time I walked home (30 min) I had my Bitcoin.  Still in 'pending' status, but I think that's normal. 

It cost me $103.95 for about $83 worth of BTC.  About $4 went to Zip Zap, $4 went to Bitinstant, and I lost $13 in the conversion to BTC (which was not obviously disclosed, probably in the fine print).  25% overhead is not a good deal IMHO.

I hope that helps someone.

Jim

I used https://localbitcoins.com/ to find someone in my area and it was a great experience. He turned out to be the head of a local Bitcoin user group! I would consider bitinstant as well, despite their mixed reviews it seems like a good product for small transfers.