I have no experience specifically with Lacoste or Polo sellers, although I would suggest an outlet store for the Polo brand...you can usually pick them up for like $20-$30 at the most. As far as suggestions for Ebay, look over the sellers feedback, I rarely buy from anyone with less than a perfect....or near perfect feedback rating. 100% is best, and make sure they have a feedback score no less than 50 or so - the more successful transactions they have completed the better. Also, ask them questions (even if you don't have one) if they are reputable they should have no problem answering and it will give you an idea of their personality. Common sense is key.
I agree with you for the most part, but I would like to add one comment about feedback scores. I have no problem buying from someone with less than perfect, but it is very important to read the comments and see why negatives are left. If someone has 99.1% feedback and 500+ feedbacks left, yet three of the negatives they did receive stated that the merchandise was not authentic, you can bet that the seller is selling/has sold more fake merchandise. However, the customers were not knowledgable enough to catch the fakes or simply were afraid to leave bad feedback. You'd be surprised at the number of sellers who sell ONLY fake merchandise and still maintain feedback ratings of above 96%. Someone who has 95% feedback and 50 transactions would be much more trustworthy if their feedback comments read something like "slow shipping" or "overcharged me for shipping".