I only need 1K Robx around 4$-4.5$

Are you talking about Robx via the t-shirt method or Robx via group funds?
Because of Roblx's new measures against the group fund method (pending Robx) prices have risen significantly.
Current rates for the t-shirt method are about 3-5 for 1k (this means the seller buys a t-shirt for whatever amount of Robx you purchase)
Current rates for the group fund method are about 6-7 for 1k