While technically it can mean almost anything .. and also varies domain to domain. If you want a vague average fuzzy number ... it's 10% of retail.
Essentially a lot of domainers have a maximum purchase price of 10% of what they think they will sell it at. Their reasoning is that they need that wide markup to maintain the rest of their portfolio renewals year after year (as well as pay their rent, etc etc).
While again that number is not in any way set in stone and veries from domainer to domainer as from domain to domain ... a as rule of thumb I think it's safe to say that the margin (% difference between "wholesale" and retail") in question goes down as the value of a domain goes up .. and conversely goes down as the price goes down.
So a $2000 retail domain might be wanted for $100 .. and a $100,000 retail domain for $50,000
Again .. those are just generalised examples .. there really is no exact science .. just as there is not real exact science on the pricing of non-liquid domains .. so at the end of the day what it means is that the person wants to buy it really cheap so that they have a wide enough margin to make it part of their portfolio WHILE being sure that portfolio pays for itself.
Remember that part of that calculation is that an average portfolio of an experienced domainer is supposed to sell about 2% of it's domains a year (again .. this is just very generalised .. the actual number varies quite significantly depending on quality and pricing). So essentially the margin between their "wholesale" purchase price and "retail" selling price needs to be able to pay for the 98% renewals of domains that don't sell in a year .. plus their expenses .. plus a profit for them to live off of ...
Please don't criticise what I said ... as I'm not saying it's right or wrong to expect to buy/sell at any particular price .. just telling you like it is ..
Fact of the matter is .. that if you are selling to almost anybody on NamePros or any other domainer focused forum/website you will almost never come close to selling your domain at full retail pricing unless it's to an outsider coming in with a very specific purchase request ... but that's under 1% of sales here .. at the most .. it's a big mistake to expect to get anything close to retail value in this forum. (Otherwise we would all be rich .. lol)