You need to distinguish between "at cost" or "close to cost" pick-ups and "I'm doing a pick-up." People who do the latter are not promising at cost; they are telling you they'll be somewhere and can pick up pins--and yes, some of them will add in a few bucks to cover their expenses (parking, gas, time, etc).
But you also need to realize that even "at cost" does not mean "the exact price on the sticker." First of all, there is tax applied; it varies by state but I think California's sales tax is relatively high, like close to 10%. Second of all, there is shipping (usually between $2 and $3, unless you buy several items and the weight of the package goes up). And finally, if you pay as goods--which is safest for you as the buyer--PayPal tacks on fees that the seller has to pay. If you do not cover those fees, the seller ends up with LESS than cost for the items, because he/she is paying those fees and not being compensated. Most sellers request that you add the couple bucks to offset those fees, so they don't do you a favor and end up losing money. Those legitimate expenses add up to $5 or $6 above the price listed on the item's barcode.