I go through thepincollection a lot and never had an issue. Most pins are going to have dozens of sellers. You should pick one with the lowest price and the best reviews. 1-2 bad reviews do not cancel 200+good. Also look at shipping. I rather pay an extra buck on the pin then see someone try to overcharge me for a stamped envelope.
For point of reference I buy and sell on eBay. (I trade tons on a monthly basis at a local meet and always need new pins, and sometimes I buy more than I am able to trade. Other times I just decide I don't want a character I collected anymore so I get rid of them.)
You can't always judge by shipping, although there are a lot of people who do overcharge. I have posted about a few of them here. I've been charged $12 for a shipment to find the seller actually paid more than that (not to mention the fact that eBay takes a minimum of 9% of whatever the seller charges for shipping and Paypal another 2.9%. A lot of sellers have that built into their shipping charge as a handling fee.) Then I've been charged $3 which I thought was reasonable to end up getting a plain envelope with the pin shoved inside and a stamp on it, as pointed out, so I basically overpaid by more than $2.00 for one pin.
If you are buying pin lots, flat rate shipping is common because the sellers often use flat rate mailers or ship first class in padded mailers and the cheapest a pin can be shipped that way is $1.64 with commercial base pricing discount from eBay. Add in their packaging costs and everything else eBay allows you to add to handling, and the cheapest flat rate shipping is around $2.00 (if the seller buys their supplies in bulk and uses eBay shipping) and in that case, the seller still loses money on the shipping because of the eBay fees, but you will still find people who complain about that. I see it in the eBay forums all the time. Buyers want free shipping because that is what eBay shoves down their throats, but what they don't realize is that sellers who offer free shipping are adding it into their item costs. So, if you buy more than one item, you may very well be overpaying. I like to see my shipping cost, not see it added into the price since I usually buy in large quantities.
Look for sellers who sell pin lots for around $2.00 a pin ($50 for 25 pins is common) and don't charge more than around $5 to ship those pins. Don't buy lots where the same seller is selling the same lot of pins over and over because you will probably get scrappers, and if it is less than $2.00 per pin on a blind lot, they often are scrappers as well. You can find new on card lots for around $2 per pin all over eBay.
I've posted about a lot of good sellers with pin lots in the last few pages of this thread. And, I am brutally honest with all aspects of the transaction. However, it is true that one or two negatives or neutrals doesn't mean the seller is bad. You need to look at the patterns, see what the comment was specifically, see how the seller responded. If the seller got hostile in their response, you probably want to stay away. Also, look at the feedback left for others by the person who left the seller that feedback. They may have a pattern of negging sellers. I've seen that and tend to just ignore those negatives knowing the seller probably didn't deserve it.
Well, that is my education of eBay lesson for the day. Hope you find what you are looking for. I leave in 11 days for my long trip and can't wait! I have a LOT of pins ready to go!!!