Other people's problems (I don't have the money right now) are not your problem. The item is not attracting many bids. OK, so it's likely to still be on the market when this person has saved up the money they need to buy it. They claim to worry someone else will buy it before them? What do you care? You just want to sell it. Really sell it, not maybe-sell-it-maybe-not-I-won't-know-for-3-months. You are tempted to take their offer because you think no-one else will buy it and this is the closest you'll get to selling it. But if that were the case, this person wouldn't need to lock you in now, would they?
If it is in fact overpriced and no-one will ever pay you 750 for it, this person should not need to buy it now even though they don't have the money (which means they are in fact almost certainly trying a scam of some sort, using your worry no-one will ever buy it.) And if it's not overpriced and there will be other buyers, then you don't need to take their offer. Either way, it's a no thanks.
To be specific, thank them for their interest and tell them you look forward to selling it to them when they've saved up the full price.