B294-“Great Britain George VI 10s Dark Blue High Value Definitive Stamp Mint/U

Title:
“Great Britain George VI 10s Dark Blue High Value Definitive Stamp – Mint/Unused”
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Category:
Collectibles → Stamps → Great Britain & Ireland → ­King George VI (1936-1952) High Values
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Condition:
Description: Mint (unused) or unused with original gum (if applicable). The image shows a clean front, good perforations, no heavy creasing. (There are some light toning spots on the back visible in your photo — note “light toning/spots on reverse, see photos”).
Grade: Very Fine to Extra Fine (assuming pretty good condition). If mounting, gum disturbance or toning present, the grade may drop to Fine/VF.
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Item Specifics:
Issue: King George VI high value 10 shilling (10s) Dark Blue (Great Britain)
Face value: 10 shillings (pre-decimal)
Colour: Dark Blue / Ultramarine as per high value series
Gum: Original/un-hinged if applicable (please state if hinged or un-hinged)
Perforation: (Please measure and state the perforation gauge, e.g., 14 × 15)
Watermark: (Please check – state upright or inverted if relevant)
Condition of back: Light toning spots present (visible)
Certification: (If you have any certificate of authenticity, mention it)
Provenance: (Optional)
Special features/varieties: None claimed (unless you have identified a variety)
Location: Kurunegala, Sri Lanka – shipping worldwide available
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Valuation & Recommended Price:
From market listings: A used example of the King George VI 10s Dark Blue is listed at US $80 (≈ £63) for a nice used example. 
Catalogue/retail values indicate mint examples of SG #478 (10s Dark Blue) for around £260 (catalogue) for mint. 
Given that your example shows some toning on the back (which reduces grade) and assuming it is unused with gum, a fair market valuation would be around £180-£220.
Recommended listing price (Buy It Now): £195
Recommended starting price if auction: £150 (with no reserve or a modest reserve of say £130)
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Listing Options:
Option A: Fixed Price – Buy It Now at £195, allow best offers.
Option B: Auction – start at £150, no reserve (or reserve at £130) and run for 7 days, listing as “Unused mint high value GB stamp – King George VI 10s”.
Timing: Choose to end the auction when UK/US buyers are active (evening UK time, weekend).
Include “Buy It Now” alternative at £195 for bidders who prefer immediate purchase.
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Shipping & Handling:
Pack well in archival-safe stamp mount with rigid backing, in a protective hard stamp case or well-cardboarded envelope.
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Photos & Description:
Upload clear high-resolution images: front, back (showing gum and toning spots), close-up of perforations and watermark (if visible).
In description include the toning spots on the back, comment that the front is clean, and state any gum condition (un-hinged/hinged).