I think that one needs to try to estimate your chances of a higher offer from this client and from a different potential end user. Admittedly you may be in the dark if the offer came through a marketplace. I think on the latter, you can at least probabilistically answer to some degree (eg if similar quality names you have held for a few years have not generated offers over $250 that is a datapoint).
The average rate of selling is somewhere around 1 per 100 domain names per year industry wide. You can, if you have been in domaining awhile, estimate how much different from average your portfolio is. I think also one should be able to estimate whether this particular domain name is significantly better than the average in terms or probability of offers and price. Let's say you have 500 names. If you ordered your names see where this particular one that you got the early offer on would lie.
Let's say you estimate that due to your picking names and promotion skills your ratio is 1 per 50 per year rather than 1 per 100. But this domain name is at best middling compared to your own portfolio, actually if you ordered not quite the mid point..
You are a little surprised that you got an offer, and it is a two word combo that not everyone would find compelling. Also, if you are honest, it is pretty similar to names in the middle of the NameBio daily list that sell in the $200 to $350 range.
So you have the choice, take $100 now (let's assume that is net so gross of $120 or whatever), or wait for more offers in future.
You might say I can't really be profitable in domaining just making $100 so I am going to hold out for $1000. Now that is about 3x the median of the industry (at least as NameBio represent it), and you find the probability of offers for this particular name is less than my average of say 1 per 50. So I could hold it for 10 years, and have approximately 10 * (1/50) *(4/5) *(1/3) of getting a $1000 offer. I base that on the 10 years, assumed probability, being slightly less good than my average so 4/5, and that I want significantly more than median price (estimate that reduces my chance of offer by factor off 3). So thee is about 1 chance in 19 that I will get offered that $1000 sometime in the next 10 years. That is $52 probabilistically. Even not taking into account the $10 per year, it does not make sense to turn down a sure $100 right now I would argue (if these numbers represented the situation for this particular name).
Now the numbers could be different. Maybe you have a single word that you think might command $25,000. Then of course the numbers would be different, even if the probabilities were even lower it could still be worth holding it. But you should be sure that your name has at least some finite chance of getting that big price. In the last year in NameBio are 69,800 .com sales. Only 291 of those sales were $25,000 or more. If I line up the domain names that really sell, is my name better than 240 out of every 241 domain names that sold? That is, is my name good enough to lead the NameBio daily list?(actually needs to do slightly better than lead a daily list).
But what about the $1000? Well many more sales there - about 11,100 of the 69,800 sales in last year are at that level. So it becomes 1 in 6 of domains that really sell. So would my domain place in the top 1/6 of the NameBio daily list. But as shown, at the $1000 level the probability of selling leaves me probabilistically with less than the rapid sale.
Sometimes refusing to move (much) on price is the right decision. I agree to make much of a living some of your names have to go for 4 figures (at least). But turning down sure money on names that statistically odds are will never in your lifetime get that price is in my mind not a route to business success. There are far more buyers willing to pay $288 for a domain name than $2888 or $28,888..
Sometimes I would argue that accepting an offer even of $100 or so is the right move. Not always, but sometimes.
JMHO.
Bob
ps To make this even longer, let me relay a personal story involving real estate, the ordinary kind. I had an unforeseen (but positive) job move after we had just purchased our first house - a huge fixer upper we had planned to spend years making nice. This was many decades ago and a very different housing and salary market. In round numbers we paid $40,000. That was a huge amount to us (we were 1 year out of being students). A real estate agent who said probably you can get $55,000 so we priced it at $49,000 because we had to move and didn't want to have a home there and live somewhere else. Within 48 hr of listing in a town where houses often went unsold literally for years we had an offer of $42,000. Our real estate agent said oh an offer came quickly, so hold out for more. Given the slow market (east coast Canada) and that we at least would not be negative even with the realtor fees, we were inclined to accept the offer, but against our better judgement and thinking wrongly that one offer meant there were more on way soon, we held our price at almost the $49,000. That buyer went away, bought a different house I think. We moved, paid funds on this house and where we were now living. Heated both through a Canadian winter.. We waited months and months and months and got not a single other offer. The stress affected our health. At this time interest rates were as I recall 16% - yes really.. It is hard to imagine if you are less than 60 that they were ever that high. The end of the story, we waited more than a year before getting another offer and finally selling at $38,000. Yup, less than the first one, less than we paid. I know I am more risk averse than many, but I really think it is important to realistically look at things probabilistically. This is definitely not an argument to always accept low $$$ offers, just be sure the quality of the name and probabilistic arguments favour you when you decide to hold out.