I want several items from the same seller...

[jamaisjedors]

Can I email them and make an offer for a "job lot"?

Not sure of the etiquette.

I want lots of bits (of a train set) and it seems such a faff to bid on them ALL individually (2 bits of track here and there, a few engines).

I'm not that bothered which bits I get, just want them from the same seller so as not to pay individual postage costs.

What do you think?

all you can do is ask.
if any of the items already have bids then the seller can not change the listing to BIN.
Also check before bidding whether they will combine the p&P for you

Thanks, will do. Didn't know that about BIN - useful to know.