40% add on to the Hammer price, well worth remembering this if bidding
I am always remembering that. There was a recent auction that I got involved with (Thanks, DickyC,

) also the Saleroom, here in the USA. Mostly an estate of real loco parts - GWR number plates etc. ("Duchess of Atholl" nameplate went for $13,000 - someone who remembered his Hornby-Dublo days?)
Anyway, my pal wanted an LGB track cleaner, and there was one in a lot of 8 LGB locos (and an empty box!) I calculated at $500, the BP and fees would be 30% (=$650) and shipping would be at least $150 for a total of $800. Resale price for all that lot might be about $1,000, with an allowance of $200 for the track cleaner, so he'd get his for less than the eBay price of $250-$350.
I won this for $550, [never go above your preset limit,

] and the invoice I just got was $714, which included a 3.75% credit card fee. I just got the shipping estimate of $200. Holy Mackerel, as the saying goes! Let's hope we can at least get our money back.
As a matter of interest, which of those (besides the track cleaner) is considered valuable in the UK?
One can do similar math on the Bachmann 4-4-0 in this latest auction, which they estimate at 180-200GBP. That's <=$254, which is pretty good compared with the recent eBay sale of the same loco, new-in-box, for $845.
But add the 20% (=$51 +VAT = $51+$10 ) for a total of $315 out of the auctioneer's door. Shipping? In my experience, at least 100 GBP, so another $125. No customs duty in the US on cheap/toy/gift items [up to $10,000] so I'm out $440. At least it has the original box, so I can sell it for $500-$600. (Despite that one at $845, most do not sell for that. MSRP was only $650. There are 2 'pre-owned' 4-4-0s on eBay.com right now for $499 and $629.) Not much profit to split with DickyC, who puts up the cash to buy it (maybe.)
Sometimes auctions do work out. There was a local auction house who said "no shipping", which put off quite a few potential buyers. My pal and I figured a nice drive to Lewes and lunch, plus picking up our spoils - what's not to like? I bought 3 freights, new-in-box, for $23, and a Kalamazoo "Empire Set" with the 4-4-0 and 2 more freights in good condition and all with original boxes. Total cost was $65. I sold the 4-4-0 for $85, and the freights for about $20 each, so the Train Fund got a little good news.