Oh Hathor, you are being so nice
If it were me I'd have just opened some whoopass, quoted my payment policy very firmly, and told him to pay now or GTFO. So, that said, you are certainly not being unreasonable thinking that 3 weeks is too long to wait. 3 weeks is too long for anyone to wait. If you can't pay within the timeline required (as listed in the auction) then you
should not bid