I've seen both sides of the issue. One the one hand you have sellers who separate every single lot even if it's one piece each. On the other hand are sellers who dump everything into one large baggie and walk away. Given the choice I prefer the overzealous packer. At least I get a bunch of extra baggies out of the deal. But surely there's some middle ground!
I've tried to strike a balance between the two and offer decent separation without eating up my cache of baggies. If a lot fits in a baggie like my 2 year old in my coat, I will add one or maybe two more similar lots so that the baggie is comfortably full. I have a theory that comfortably filling a bag allows for less moving and therefore less scratching. Less baggies also shaves off weight. Fill the baggie too full though and you risk gouging due to the tight quarters.
|Big items in the big Ziploc, little baggies to follow.|
When mixing lots, I try to keep things similar enough. Bricks would go together, or slopes with the bricks. But I suppose it would make more sense to put two very distinct lots together. A handful of blue technic pins with a handful of yellow modified bricks. But be careful.
There are a few parts that should always be kept protected. Those with large faces such as 1x6x5 panels and bricks, and transparent pieces can easily scratch. I keep these separate, even bagging individually if I feel it is warranted.
I try to separate colors as best I can so that a pile of black 1x1 plates is not mixed in with a pile of black 1x2 plates. If there's only 3 pieces per lot though, I'm sorry, you're probably getting all of your reddish brown 1x plates in the same baggie.
|Lot mixing examples|
It used to be that I'd throw all the loose baggies in the mailer and call it good. However more and more I'll stuff smaller baggies into bigger ones to use up space. If there a lot of baggies then everything goes into one bigger zip baggie before the mailer.
One other option I've offered to buyers is to stack or otherwise connect some of the bricks. Sometimes I can get a smaller package and a cheaper rate this way, especially internationally. Or in the case of the 28 large tires and matching rims, allowed me to pack a small flat rate priority box just perfectly.
Every order is different. Think creatively and you can pack well, pack safe, and ship cheap.