If you’re not a really, really good negotiator, temporarily trading your amps down and then back up is going to cost quite a bit, i.e. loss of value on items you’re selling, shipping costs, Audiogon fees, etc. Those amps are heavy and all that buying and selling is time consuming.
I think it would be cheaper to take advantage of low interest rates and get a loan to finance the speaker upgrade.
Maybe put them on a new credit card that offers zero interest for 6 months or something.