Like any other brandable, it has no value except what someone is willing to pay. I'd refer them to SuperSaleTraining.com, which is a domain in a similar vein which sold a few months ago for $1,100 (3 June at AfterNIC in case you need a specific reference.) $800 is not too out of line for that domain, in my opinion, but perhaps you can notch it up a little. You might suggest that you had been considering putting it out to auction at the start of the year, since as a sales-related brandable it would be an attractive acquisition for a marketing company, and Q1 is when such companies tend to spend their annual budgets for one-off expenditures (this is true btw, but I would never advocate tossing the bird back into the bush, so to speak. It's just an offhand comment to make to a marketing company who wants your domain, to let them know you know a bit about their business and that you know where and when to find another buyer if they back off.)
Anyway, as I too often am guilty of doing, there's no real appraisal in this appraisal post, but the short version of the above is that I think that $800 is a pretty good price, but you have a little room there between the similar niche sale and upcoming budget "recharge" for the marketing industry, that you may be able to edge a little more out of it.
Frank