[My Name is Frank]

I would think the bird in the hand, would you not have to pay a percentage on top of anything to the auction house at auction, so the offer in effect is more than £8000?

[fusion0001]

Would really depend on the artwork, but something to keep in mind is that if this is the first proposed estimate by Sotheby's, then those are typically significantly below actual value of the work, since the auction house wants to make sure they have as close as 100% sell-through and hence their recommended low estimate is usually far below market value.

[startimeash]

Do you have a particular thing that you want to do with the money? If so, and if £8k will cover it, then take that. If it's just going to sit in the bank, and you're treating it as a nest egg, I'd be inclined to go to auction. If £8k would give me a holiday of a lifetime, for example, then I'd prefer to know that was paid for, rather than stress about the "what if's" of going to auction. That said, both other posters make good points. For me, though, if you have it earmarked for a particular project or use, take the cash.

[Daniel Silk]

I guess you also need to think about if it went to auction that could be month away, and that could mean the value going up or down?
Also like others have said, if you plan to buy another piece of art with that money, do you really need it now! Will that new piece be sold out or gone up in value in a few months time?

[Riotcops]

If I needed the money right away I'd sell. If not, it would head to auction. You're essentially betting £800 for an unknown upside but based on the high estimate it would be 4.5x or potentially much higher. If one person is willing to pay £8k right now I'd presume there will be many more.

[drbf]

Sotheby's will give you three values.
1. Auction estimate
2. Insurance valuation
3. Retail valuation.
Get these three amounts and decide what to do.